x-www-form-urlencoded meets Serde

12 files changed, 1761 insertions, 0 deletions

diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..a9d37c5
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+target
+Cargo.lock
diff --git a/.travis.yml b/.travis.yml
new file mode 100644
index 0000000..4ecd9b6
--- /dev/null
+++ b/.travis.yml
@@ -0,0 +1,5 @@
+language: rust
+rust:
+  - nightly
+  - beta
+  - stable
diff --git a/Cargo.toml b/Cargo.toml
new file mode 100644
index 0000000..a5098d9
--- /dev/null
+++ b/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+name = "serde_urlencoded"
+version = "0.1.0"
+authors = ["Anthony Ramine <n.oxyde@gmail.com>"]
+license = "MIT/Apache-2.0"
+repository = "https://github.com/nox/serde_urlencoded"
+documentation = "https://docs.rs/crate/serde_urlencoded"
+description = "`x-www-form-urlencoded` meets Serde"
+keywords = ["serde", "serialization", "urlencoded"]
+
+[dependencies]
+serde = "0.8.7"
+url = "1.0.0"
diff --git a/LICENSE-APACHE b/LICENSE-APACHE
new file mode 100644
index 0000000..1b5ec8b
--- /dev/null
+++ b/LICENSE-APACHE
@@ -0,0 +1,176 @@
+                              Apache License
+                        Version 2.0, January 2004
+                     http://www.apache.org/licenses/
+
+TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
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+   any Contribution intentionally submitted for inclusion in the Work
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+   Notwithstanding the above, nothing herein shall supersede or modify
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+6. Trademarks. This License does not grant permission to use the trade
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+END OF TERMS AND CONDITIONS
diff --git a/LICENSE-MIT b/LICENSE-MIT
new file mode 100644
index 0000000..39f6303
--- /dev/null
+++ b/LICENSE-MIT
@@ -0,0 +1,25 @@
+Copyright (c) 2016 Anthony Ramine
+
+Permission is hereby granted, free of charge, to any
+person obtaining a copy of this software and associated
+documentation files (the "Software"), to deal in the
+Software without restriction, including without
+limitation the rights to use, copy, modify, merge,
+publish, distribute, sublicense, and/or sell copies of
+the Software, and to permit persons to whom the Software
+is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice
+shall be included in all copies or substantial portions
+of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF
+ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
+TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
+PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT
+SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
+CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR
+IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..0c9fe8f
--- /dev/null
+++ b/README.md
@@ -0,0 +1,50 @@
+`x-www-form-urlencoded` meets Serde
+===================================
+
+This crate is a Rust library for serialising to and deserialising from
+the [`application/x-www-form-urlencoded`][urlencoded] format. It is built
+upon [Serde], a high performance generic serialization framework and [rust-url],
+a URL parser for Rust.
+
+[rust-url]: https://github.com/servo/rust-url
+[Serde]: https://github.com/serde-rs/serde
+[urlencoded]: https://url.spec.whatwg.org/#application/x-www-form-urlencoded
+
+Installation
+============
+
+This crate works with Cargo and can be found on
+[crates.io] with a `Cargo.toml` like:
+
+```toml
+[dependencies]
+serde_urlencoded = "0.1"
+```
+
+[crates.io]: https://crates.io/crates/serde_urlencoded
+
+## Getting help
+
+Serde developers live in the #serde channel on
+[`irc.mozilla.org`](https://wiki.mozilla.org/IRC) and most rust-url developers
+live in the #servo one. The #rust channel is also a good resource with generally
+faster response time but less specific knowledge about Serde, rust-url or this
+crate. If IRC is not your thing, we are happy to respond to [GitHub
+issues](https://github.com/nox/serde_urlencoded/issues/new) as well.
+
+## License
+
+Serde is licensed under either of
+
+ * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
+   http://www.apache.org/licenses/LICENSE-2.0)
+ * MIT license ([LICENSE-MIT](LICENSE-MIT) or
+   http://opensource.org/licenses/MIT)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in Serde by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.
diff --git a/src/de.rs b/src/de.rs
new file mode 100644
index 0000000..ec84ff2
--- /dev/null
+++ b/src/de.rs
@@ -0,0 +1,90 @@
+//! Deserialization support for the `application/x-www-form-urlencoded` format.
+
+use serde::de;
+use serde::de::value::MapDeserializer;
+use std::borrow::Cow;
+use std::marker::PhantomData;
+use url::form_urlencoded::Parse;
+
+pub use serde::de::value::Error;
+
+/// A deserializer for the `application/x-www-form-urlencoded` format.
+///
+/// * Supported top-level outputs are structs, maps and sequences of pairs,
+///   with or without a given length.
+///
+/// * Main `deserialize` methods defers to `deserialize_map`.
+///
+/// * Everything else but `deserialize_seq` and `deserialize_seq_fixed_size`
+///   defers to `deserialize`.
+pub struct Deserializer<'a, T> {
+    deserializer: MapDeserializer<Parse<'a>, Cow<'a, str>, Cow<'a, str>, Error>,
+    marker: PhantomData<T>,
+}
+
+impl<'a, T> de::Deserializer for Deserializer<'a, T>
+{
+    type Error = Error;
+
+    fn deserialize<V>(
+            &mut self, visitor: V)
+            -> Result<V::Value, Self::Error>
+        where V: de::Visitor,
+    {
+        self.deserialize_map(visitor)
+    }
+
+    fn deserialize_map<V>(
+            &mut self, mut visitor: V)
+            -> Result<V::Value, Self::Error>
+        where V: de::Visitor,
+    {
+        visitor.visit_map(&mut self.deserializer)
+    }
+
+    fn deserialize_seq<V>(
+            &mut self, mut visitor: V)
+            -> Result<V::Value, Self::Error>
+        where V: de::Visitor,
+    {
+        visitor.visit_seq(&mut self.deserializer)
+    }
+
+    fn deserialize_seq_fixed_size<V>(
+            &mut self, _len: usize, mut visitor: V)
+            -> Result<V::Value, Self::Error>
+        where V: de::Visitor
+    {
+        visitor.visit_seq(&mut self.deserializer)
+    }
+
+    forward_to_deserialize! {
+        bool
+        usize
+        u8
+        u16
+        u32
+        u64
+        isize
+        i8
+        i16
+        i32
+        i64
+        f32
+        f64
+        char
+        str
+        string
+        unit
+        option
+        bytes
+        unit_struct
+        newtype_struct
+        tuple_struct
+        struct
+        struct_field
+        tuple
+        enum
+        ignored_any
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..95a0ad6
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,11 @@
+//! `x-www-form-urlencoded` meets Serde
+
+#[macro_use]
+extern crate serde;
+extern crate url;
+
+pub mod de;
+pub mod ser;
+
+pub use de::Deserializer;
+pub use ser::Serializer;
diff --git a/src/ser/key.rs b/src/ser/key.rs
new file mode 100644
index 0000000..a26c563
--- /dev/null
+++ b/src/ser/key.rs
@@ -0,0 +1,292 @@
+use serde::{Serialize, Serializer};
+use ser::Error;
+use std::borrow::Cow;
+use std::str;
+
+pub struct MapKeySerializer<'key>(&'key mut Option<Cow<'static, str>>);
+
+impl<'key> MapKeySerializer<'key> {
+    pub fn new(output: &'key mut Option<Cow<'static, str>>) -> Self {
+        MapKeySerializer(output)
+    }
+
+    fn set_key<T>(&mut self, key: T) -> Result<(), Error>
+        where T: Into<Cow<'static, str>>
+    {
+        *self.0 = Some(key.into());
+        Ok(())
+    }
+}
+
+impl<'key> Serializer for MapKeySerializer<'key> {
+    type Error = Error;
+    type SeqState = ();
+    type TupleState = ();
+    type TupleStructState = ();
+    type TupleVariantState = ();
+    type MapState = ();
+    type StructState = ();
+    type StructVariantState = ();
+
+    fn serialize_bool(&mut self, _v: bool) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_isize(&mut self, v: isize) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_i8(&mut self, v: i8) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_i16(&mut self, v: i16) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_i32(&mut self, v: i32) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_i64(&mut self, v: i64) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_usize(&mut self, v: usize) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_u8(&mut self, v: u8) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_u16(&mut self, v: u16) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_u32(&mut self, v: u32) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_u64(&mut self, v: u64) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_f32(&mut self, v: f32) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_f64(&mut self, v: f64) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_char(&mut self, v: char) -> Result<(), Error> {
+        self.set_key(v.to_string())
+    }
+
+    fn serialize_str(&mut self, value: &str) -> Result<(), Error> {
+        self.set_key(String::from(value))
+    }
+
+    fn serialize_bytes(&mut self, value: &[u8]) -> Result<(), Error> {
+        match str::from_utf8(value) {
+            Ok(value) => self.set_key(String::from(value)),
+            Err(err) => Err(Error::Utf8(err)),
+        }
+    }
+
+    fn serialize_unit(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_unit_struct(
+            &mut self, name: &'static str)
+            -> Result<(), Error> {
+        self.set_key(name)
+    }
+
+    fn serialize_unit_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            variant: &'static str)
+            -> Result<(), Error> {
+        self.set_key(variant)
+    }
+
+    fn serialize_newtype_struct<T>(
+            &mut self, _name: &'static str, value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T>(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_none(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_some<T>(&mut self, _value: T) -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_seq(&mut self, _len: Option<usize>) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_seq_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_seq_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_seq_fixed_size(&mut self, _size: usize) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple(&mut self, _len: usize) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_struct(
+            &mut self, _name: &'static str, _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_struct_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_struct_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_variant_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_tuple_variant_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_map(&mut self, _len: Option<usize>) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_map_key<T>(
+            &mut self, _state: &mut (), _key: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_map_value<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_map_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct(
+            &mut self, _name: &'static str, _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct_elt<T>(
+            &mut self, _state: &mut (), _key: &'static str, _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct_variant_elt<T>(
+            &mut self, _state: &mut (), _key: &'static str, _value: T)
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+
+    fn serialize_struct_variant_end(
+            &mut self, _state: ())
+            -> Result<(), Error> {
+        Err(Error::unsupported_key())
+    }
+}
+
+impl Error {
+    fn unsupported_key() -> Self {
+        Error::Custom("unsupported key".into())
+    }
+}
diff --git a/src/ser/mod.rs b/src/ser/mod.rs
new file mode 100644
index 0000000..387766d
--- /dev/null
+++ b/src/ser/mod.rs
@@ -0,0 +1,445 @@
+//! Serialization support for the `application/x-www-form-urlencoded` format.
+
+mod key;
+mod pair;
+mod value;
+
+use serde::ser;
+use std::borrow::Cow;
+use std::error;
+use std::fmt;
+use std::str;
+use url::form_urlencoded;
+
+/// A serializer for the `application/x-www-form-urlencoded` format.
+///
+/// * Supported top-level inputs are structs, maps and sequences of pairs,
+///   with or without a given length.
+///
+/// * Supported keys and values are integers, bytes (if convertible to strings),
+///   unit structs and unit variants.
+///
+/// * Newtype structs defer to their inner values.
+pub struct Serializer<T>(form_urlencoded::Serializer<T>)
+    where T: form_urlencoded::Target;
+
+/// Errors returned during serializing to `application/x-www-form-urlencoded`.
+#[derive(Clone, Debug)]
+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> {
+        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),
+        }
+    }
+}
+
+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),
+        }
+    }
+
+    /// 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::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())
+    }
+}
+
+/// State used when serializing sequences.
+pub struct SeqState(());
+
+/// State used when serializing tuples.
+pub struct TupleState(());
+
+/// State used when serializing tuple structs.
+pub struct TupleStructState(());
+
+/// State used when serializing tuple variants.
+pub struct TupleVariantState(());
+
+/// State used when serializing maps.
+pub struct MapState(Option<Cow<'static, str>>);
+
+/// State used when serializing structs.
+pub struct StructState(());
+
+/// State used when serializing struct variants.
+pub struct StructVariantState(());
+
+impl<Target: form_urlencoded::Target> ser::Serializer for Serializer<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())
+    }
+
+    /// Returns an error.
+    fn serialize_i8(&mut self, _v: i8) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_i16(&mut self, _v: i16) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_i32(&mut self, _v: i32) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_i64(&mut self, _v: i64) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_usize(&mut self, _v: usize) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_u8(&mut self, _v: u8) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_u16(&mut self, _v: u16) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_u32(&mut self, _v: u32) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_u64(&mut self, _v: u64) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_f32(&mut self, _v: f32) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_f64(&mut self, _v: f64) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_char(&mut self, _v: char) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_str(&mut self, _value: &str) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_bytes(&mut self, _value: &[u8]) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_unit(&mut self) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_unit_struct(
+            &mut self, _name: &'static str)
+            -> Result<(), 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> {
+        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
+    {
+        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
+    {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_none(&mut self) -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_some<T>(&mut self, _value: T) -> Result<(), Error>
+        where T: ser::Serialize
+    {
+        Err(Error::top_level())
+    }
+
+    /// Begins to serialize a sequence, given length (if any) is ignored.
+    fn serialize_seq(
+            &mut self, _len: Option<usize>)
+            -> Result<SeqState, Error> {
+        Ok(SeqState(()))
+    }
+
+    /// 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(&mut self.0))
+    }
+
+    /// 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(()))
+    }
+
+    /// Returns an error.
+    fn serialize_tuple(&mut self, _len: usize) -> Result<TupleState, 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
+    {
+        Err(Error::top_level())
+    }
+
+    /// Returns an error.
+    fn serialize_tuple_end(&mut self, _state: TupleState) -> Result<(), 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())
+    }
+
+    /// 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())
+    }
+
+    /// Returns an error.
+    fn serialize_tuple_struct_end(
+            &mut self, _state: TupleStructState)
+            -> Result<(), 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())
+    }
+
+    /// 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())
+    }
+
+    /// Returns an error.
+    fn serialize_tuple_variant_end(
+            &mut self, _state: TupleVariantState)
+            -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+
+    /// Begins to serialize a map, given length (if any) is ignored.
+    fn serialize_map(
+            &mut self, _len: Option<usize>)
+            -> Result<MapState, Error> {
+        Ok(MapState(None))
+    }
+
+    /// 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.0))
+    }
+
+    /// 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.0, &mut self.0));
+        value.serialize(&mut value_serializer)
+    }
+
+    /// Finishes serializing a map.
+    fn serialize_map_end(&mut self, _state: MapState) -> Result<(), Error> {
+        Ok(())
+    }
+
+    /// Begins to serialize a struct, given length is ignored.
+    fn serialize_struct(
+            &mut self, _name: &'static str, _len: usize)
+            -> Result<StructState, Error> {
+        Ok(StructState(()))
+    }
+
+    /// 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, &mut self.0).unwrap();
+        value.serialize(&mut value_serializer)
+    }
+
+    /// Finishes serializing a struct.
+    fn serialize_struct_end(&mut self, _state: StructState)
+                            -> Result<(), Error> {
+        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())
+    }
+
+    /// 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())
+    }
+
+    /// Returns an error.
+    fn serialize_struct_variant_end(
+            &mut self, _state: StructVariantState)
+            -> Result<(), Error> {
+        Err(Error::top_level())
+    }
+}
+
+impl Error {
+    fn top_level() -> Self {
+        Error::Custom(
+            "top-level serializer supports only maps and structs".into())
+    }
+}
diff --git a/src/ser/pair.rs b/src/ser/pair.rs
new file mode 100644
index 0000000..ae42db0
--- /dev/null
+++ b/src/ser/pair.rs
@@ -0,0 +1,330 @@
+use ser::{Error, key, value};
+use serde::{Serialize, Serializer};
+use std::borrow::Cow;
+use url::form_urlencoded;
+
+pub struct PairSerializer<'target, Target>(
+        &'target mut form_urlencoded::Serializer<Target>)
+    where Target: 'target + form_urlencoded::Target;
+
+impl<'target, Target> PairSerializer<'target, Target>
+    where Target: 'target + form_urlencoded::Target
+{
+    pub fn new(
+            serializer: &'target mut form_urlencoded::Serializer<Target>)
+            -> Self {
+        PairSerializer(serializer)
+    }
+}
+
+pub struct TupleState(Option<Option<Cow<'static, str>>>);
+pub struct TupleStructState(TupleState);
+
+impl<'target, Target> Serializer for PairSerializer<'target, Target>
+    where Target: 'target + form_urlencoded::Target
+{
+    type Error = Error;
+    type SeqState = ();
+    type TupleState = TupleState;
+    type TupleStructState = TupleStructState;
+    type TupleVariantState = ();
+    type MapState = ();
+    type StructState = ();
+    type StructVariantState = ();
+
+    fn serialize_bool(&mut self, _v: bool) -> Result<(), Error> {
+        Err(Error::Custom("booleans are not supported values".into()))
+    }
+
+    fn serialize_isize(&mut self, _v: isize) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_i8(&mut self, _v: i8) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_i16(&mut self, _v: i16) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_i32(&mut self, _v: i32) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_i64(&mut self, _v: i64) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_usize(&mut self, _v: usize) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_u8(&mut self, _v: u8) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_u16(&mut self, _v: u16) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_u32(&mut self, _v: u32) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_u64(&mut self, _v: u64) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_f32(&mut self, _v: f32) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_f64(&mut self, _v: f64) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_char(&mut self, _v: char) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_str(&mut self, _value: &str) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_bytes(&mut self, _value: &[u8]) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_unit(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_unit_struct(
+            &mut self, _name: &'static str)
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_unit_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str)
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_newtype_struct<T>(
+            &mut self, _name: &'static str, value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T>(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_none(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_some<T>(&mut self, _value: T) -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_seq(&mut self, _len: Option<usize>)
+                     -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_seq_elt<T>(&mut self, _state: &mut (), _value: T)
+                            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_seq_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_seq_fixed_size(&mut self, _size: usize)
+                                -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_tuple(&mut self, len: usize) -> Result<TupleState, Error> {
+        if len == 2 {
+            Ok(TupleState(None))
+        } else {
+            Err(Error::unsupported_pair())
+        }
+    }
+
+    fn serialize_tuple_elt<T>(
+            &mut self, state: &mut TupleState, value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        match state.0.take() {
+            None => {
+                let mut key = None;
+                {
+                    let mut key_serializer =
+                        key::MapKeySerializer::new(&mut key);
+                    try!(value.serialize(&mut key_serializer));
+                }
+                state.0 = Some(key);
+                Ok(())
+            },
+            Some(ref mut key) => {
+                {
+                    let mut value_serializer =
+                        value::ValueSerializer::new(key, &mut self.0).unwrap();
+                    try!(value.serialize(&mut value_serializer));
+                }
+                state.0 = Some(None);
+                Ok(())
+            }
+        }
+    }
+
+    fn serialize_tuple_end(&mut self, _state: TupleState) -> Result<(), Error> {
+        Ok(())
+    }
+
+    fn serialize_tuple_struct(
+            &mut self, _name: &'static str, len: usize)
+            -> Result<TupleStructState, Error> {
+        self.serialize_tuple(len).map(TupleStructState)
+    }
+
+    fn serialize_tuple_struct_elt<T>(
+            &mut self, state: &mut TupleStructState, value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        self.serialize_tuple_elt(&mut state.0, value)
+    }
+
+    fn serialize_tuple_struct_end(
+            &mut self, _state: TupleStructState)
+            -> Result<(), Error> {
+        Ok(())
+    }
+
+    fn serialize_tuple_variant(
+        &mut self,
+        _name: &'static str,
+        _variant_index: usize,
+        _variant: &'static str,
+        _len: usize)
+        -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_tuple_variant_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_tuple_variant_end(
+            &mut self, _state: ())
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_map(
+            &mut self, _len: Option<usize>)
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_map_key<T>(
+            &mut self, _state: &mut (), _key: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_map_value<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_map_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_struct(&mut self, _name: &'static str, _len: usize)
+                        -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_struct_elt<T>(
+            &mut self,
+            _state: &mut (),
+            _key: &'static str,
+            _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_struct_end(
+            &mut self, _state: ())
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_struct_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+    fn serialize_struct_variant_elt<T>(
+            &mut self,
+            _state: &mut (),
+            _key: &'static str,
+            _value: T)
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+
+    fn serialize_struct_variant_end(
+            &mut self, _state: ())
+            -> Result<(), Error> {
+        Err(Error::unsupported_pair())
+    }
+}
+
+impl Error {    
+    fn unsupported_pair() -> Self {
+        Error::Custom("unsupported pair".into())
+    }
+}
diff --git a/src/ser/value.rs b/src/ser/value.rs
new file mode 100644
index 0000000..39d3af0
--- /dev/null
+++ b/src/ser/value.rs
@@ -0,0 +1,322 @@
+use ser::Error;
+use serde::{Serialize, Serializer};
+use std::borrow::Cow;
+use std::str;
+use url::form_urlencoded;
+
+pub struct ValueSerializer<'key, 'target, Target>
+    where Target: 'target + form_urlencoded::Target
+{
+    key: &'key mut Option<Cow<'static, str>>,
+    serializer: &'target mut form_urlencoded::Serializer<Target>
+}
+
+impl<'key, 'target, Target> ValueSerializer<'key, 'target, Target>
+    where Target: 'target + form_urlencoded::Target
+{
+    pub fn new(
+            key: &'key mut Option<Cow<'static, str>>,
+            serializer: &'target mut form_urlencoded::Serializer<Target>)
+            -> Result<Self, Error> {
+        if key.is_some() {
+            Ok(ValueSerializer {
+                key: key,
+                serializer: serializer,
+            })
+        } else {
+            Err(Error::no_key())
+        }
+    }
+
+    fn append_pair(&mut self, value: &str) -> Result<(), Error> {
+        if let Some(key) = self.key.take() {
+            self.serializer.append_pair(&key, value);
+            Ok(())
+        } else {
+            Err(Error::no_key())
+        }
+    }
+}
+
+impl<'key, 'target, Target> Serializer
+    for ValueSerializer<'key, 'target, Target>
+    where Target: 'target + form_urlencoded::Target
+{
+    type Error = Error;
+    type SeqState = ();
+    type TupleState = ();
+    type TupleStructState = ();
+    type TupleVariantState = ();
+    type MapState = ();
+    type StructState = ();
+    type StructVariantState = ();
+
+    fn serialize_bool(&mut self, _v: bool) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_isize(&mut self, v: isize) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_i8(&mut self, v: i8) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_i16(&mut self, v: i16) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_i32(&mut self, v: i32) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_i64(&mut self, v: i64) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_usize(&mut self, v: usize) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_u8(&mut self, v: u8) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_u16(&mut self, v: u16) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_u32(&mut self, v: u32) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_u64(&mut self, v: u64) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_f32(&mut self, v: f32) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_f64(&mut self, v: f64) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_char(&mut self, v: char) -> Result<(), Error> {
+        self.append_pair(&v.to_string())
+    }
+
+    fn serialize_str(&mut self, value: &str) -> Result<(), Error> {
+        self.append_pair(value)
+    }
+
+    fn serialize_bytes(&mut self, value: &[u8]) -> Result<(), Error> {
+        match str::from_utf8(value) {
+            Ok(value) => self.append_pair(value),
+            Err(err) => Err(Error::Utf8(err)),
+        }
+    }
+
+    fn serialize_unit(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_unit_struct(
+            &mut self, name: &'static str)
+            -> Result<(), Error> {
+        self.append_pair(name)
+    }
+
+    fn serialize_unit_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            variant: &'static str)
+            -> Result<(), Error> {
+        self.append_pair(variant)
+    }
+
+    fn serialize_newtype_struct<T>(
+            &mut self, _name: &'static str, value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        value.serialize(self)
+    }
+
+    fn serialize_newtype_variant<T>(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_none(&mut self) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_some<T>(&mut self, _value: T) -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_seq(&mut self, _len: Option<usize>) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_seq_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_seq_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_seq_fixed_size(&mut self, _size: usize) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple(&mut self, _len: usize) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_struct(
+            &mut self, _name: &'static str, _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_struct_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_struct_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_variant(
+        &mut self,
+        _name: &'static str,
+        _variant_index: usize,
+        _variant: &'static str,
+        _len: usize)
+        -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_variant_elt<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_tuple_variant_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_map(&mut self, _len: Option<usize>) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_map_key<T>(
+            &mut self, _state: &mut (), _key: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_map_value<T>(
+            &mut self, _state: &mut (), _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_map_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_struct(
+            &mut self, _name: &'static str, _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_struct_elt<T>(
+            &mut self,
+            _state: &mut (),
+            _key: &'static str,
+            _value: T)
+            -> Result<(), Error>
+        where T: Serialize
+    {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_struct_end(&mut self, _state: ()) -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_struct_variant(
+            &mut self,
+            _name: &'static str,
+            _variant_index: usize,
+            _variant: &'static str,
+            _len: usize)
+            -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+    fn serialize_struct_variant_elt<T>(
+            &mut self, _state: &mut (), _key: &'static str, _value: T)
+            -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+
+    fn serialize_struct_variant_end(
+            &mut self, _state: ())
+            -> Result<(), Error> {
+        Err(Error::unsupported_value())
+    }
+}
+
+impl Error {
+    fn no_key() -> Self {
+        Error::Custom(
+            "tried to serialize a value before serializing key".into())
+    }
+
+    fn unsupported_value() -> Self {
+        Error::Custom("unsupported value".into())
+    }
+}