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    //! Deserialization support for the `application/x-www-form-urlencoded` format.

    use serde::de::Error as de_Error;
    use serde::de::{
        self, DeserializeSeed, EnumAccess, IntoDeserializer, VariantAccess, Visitor,
    };

    use serde::de::value::MapDeserializer;
    use std::borrow::Cow;
    use std::io::Read;
    use url::form_urlencoded::parse;
    use url::form_urlencoded::Parse as UrlEncodedParse;

    #[doc(inline)]
    pub use serde::de::value::Error;

    /// Deserializes a `application/x-wwww-url-encoded` value from a `&[u8]`.
    ///
    /// ```ignore
    /// let meal = vec![
    ///     ("bread".to_owned(), "baguette".to_owned()),
    ///     ("cheese".to_owned(), "comté".to_owned()),
    ///     ("meat".to_owned(), "ham".to_owned()),
    ///     ("fat".to_owned(), "butter".to_owned()),
    /// ];
    ///
    /// assert_eq!(
    ///     serde_urlencoded::from_bytes::<Vec<(String, String)>>(
    ///         b"bread=baguette&cheese=comt%C3%A9&meat=ham&fat=butter"),
    ///     Ok(meal));
    /// ```
    pub fn from_bytes<'de, T>(input: &'de [u8]) -> Result<T, Error>
    where
        T: de::Deserialize<'de>,
    {
        T::deserialize(Deserializer::new(parse(input)))
    }

    /// Deserializes a `application/x-wwww-url-encoded` value from a `&str`.
    ///
    /// ```ignore
    /// let meal = vec![
    ///     ("bread".to_owned(), "baguette".to_owned()),
    ///     ("cheese".to_owned(), "comté".to_owned()),
    ///     ("meat".to_owned(), "ham".to_owned()),
    ///     ("fat".to_owned(), "butter".to_owned()),
    /// ];
    ///
    /// assert_eq!(
    ///     serde_urlencoded::from_str::<Vec<(String, String)>>(
    ///         "bread=baguette&cheese=comt%C3%A9&meat=ham&fat=butter"),
    ///     Ok(meal));
    /// ```
    pub fn from_str<'de, T>(input: &'de str) -> Result<T, Error>
    where
        T: de::Deserialize<'de>,
    {
        from_bytes(input.as_bytes())
    }

    #[allow(dead_code)]
    /// Convenience function that reads all bytes from `reader` and deserializes
    /// them with `from_bytes`.
    pub fn from_reader<T, R>(mut reader: R) -> Result<T, Error>
    where
        T: de::DeserializeOwned,
        R: Read,
    {
        let mut buf = vec![];
        reader
            .read_to_end(&mut buf)
            .map_err(|e| de::Error::custom(format_args!("could not read input: {}", e)))?;
        from_bytes(&buf)
    }

    /// 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<'de> {
        inner: MapDeserializer<'de, PartIterator<'de>, Error>,
    }

    impl<'de> Deserializer<'de> {
        /// Returns a new `Deserializer`.
        pub fn new(parser: UrlEncodedParse<'de>) -> Self {
            Deserializer {
                inner: MapDeserializer::new(PartIterator(parser)),
            }
        }
    }

    impl<'de> de::Deserializer<'de> for Deserializer<'de> {
        type Error = Error;

        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            self.deserialize_map(visitor)
        }

        fn deserialize_map<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            visitor.visit_map(self.inner)
        }

        fn deserialize_seq<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            visitor.visit_seq(self.inner)
        }

        fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            self.inner.end()?;
            visitor.visit_unit()
        }

        forward_to_deserialize_any! {
            bool
            u8
            u16
            u32
            u64
            i8
            i16
            i32
            i64
            f32
            f64
            char
            str
            string
            option
            bytes
            byte_buf
            unit_struct
            newtype_struct
            tuple_struct
            struct
            identifier
            tuple
            enum
            ignored_any
        }
    }

    struct PartIterator<'de>(UrlEncodedParse<'de>);

    impl<'de> Iterator for PartIterator<'de> {
        type Item = (Part<'de>, Part<'de>);

        fn next(&mut self) -> Option<Self::Item> {
            self.0.next().map(|(k, v)| (Part(k), Part(v)))
        }
    }

    struct Part<'de>(Cow<'de, str>);

    impl<'de> IntoDeserializer<'de> for Part<'de> {
        type Deserializer = Self;

        fn into_deserializer(self) -> Self::Deserializer {
            self
        }
    }

    macro_rules! forward_parsed_value {
        ($($ty:ident => $method:ident,)*) => {
            $(
                fn $method<V>(self, visitor: V) -> Result<V::Value, Self::Error>
                    where V: de::Visitor<'de>
                {
                    match self.0.parse::<$ty>() {
                        Ok(val) => val.into_deserializer().$method(visitor),
                        Err(e) => Err(de::Error::custom(e))
                    }
                }
            )*
        }
    }

    impl<'de> de::Deserializer<'de> for Part<'de> {
        type Error = Error;

        fn deserialize_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            self.0.into_deserializer().deserialize_any(visitor)
        }

        fn deserialize_option<V>(self, visitor: V) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            visitor.visit_some(self)
        }

        fn deserialize_enum<V>(
            self, _name: &'static str, _variants: &'static [&'static str], visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: de::Visitor<'de>,
        {
            visitor.visit_enum(ValueEnumAccess { value: self.0 })
        }

        forward_to_deserialize_any! {
            char
            str
            string
            unit
            bytes
            byte_buf
            unit_struct
            newtype_struct
            tuple_struct
            struct
            identifier
            tuple
            ignored_any
            seq
            map
        }

        forward_parsed_value! {
            bool => deserialize_bool,
            u8 => deserialize_u8,
            u16 => deserialize_u16,
            u32 => deserialize_u32,
            u64 => deserialize_u64,
            i8 => deserialize_i8,
            i16 => deserialize_i16,
            i32 => deserialize_i32,
            i64 => deserialize_i64,
            f32 => deserialize_f32,
            f64 => deserialize_f64,
        }
    }

    /// Provides access to a keyword which can be deserialized into an enum variant. The enum variant
    /// must be a unit variant, otherwise deserialization will fail.
    struct ValueEnumAccess<'de> {
        value: Cow<'de, str>,
    }

    impl<'de> EnumAccess<'de> for ValueEnumAccess<'de> {
        type Error = Error;
        type Variant = UnitOnlyVariantAccess;

        fn variant_seed<V>(self, seed: V) -> Result<(V::Value, Self::Variant), Self::Error>
        where
            V: DeserializeSeed<'de>,
        {
            let variant = seed.deserialize(self.value.into_deserializer())?;
            Ok((variant, UnitOnlyVariantAccess))
        }
    }

    /// A visitor for deserializing the contents of the enum variant. As we only support
    /// `unit_variant`, all other variant types will return an error.
    struct UnitOnlyVariantAccess;

    impl<'de> VariantAccess<'de> for UnitOnlyVariantAccess {
        type Error = Error;

        fn unit_variant(self) -> Result<(), Self::Error> {
            Ok(())
        }

        fn newtype_variant_seed<T>(self, _seed: T) -> Result<T::Value, Self::Error>
        where
            T: DeserializeSeed<'de>,
        {
            Err(Error::custom("expected unit variant"))
        }

        fn tuple_variant<V>(self, _len: usize, _visitor: V) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>,
        {
            Err(Error::custom("expected unit variant"))
        }

        fn struct_variant<V>(
            self, _fields: &'static [&'static str], _visitor: V,
        ) -> Result<V::Value, Self::Error>
        where
            V: Visitor<'de>,
        {
            Err(Error::custom("expected unit variant"))
        }
    }

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