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    //! This module contains `Impossible` serializer and its implementations.

    use lib::*;

    use ser::{
        self, Serialize, SerializeMap, SerializeSeq, SerializeStruct, SerializeStructVariant,
        SerializeTuple, SerializeTupleStruct, SerializeTupleVariant,
    };

    /// Helper type for implementing a `Serializer` that does not support
    /// serializing one of the compound types.
    ///
    /// This type cannot be instantiated, but implements every one of the traits
    /// corresponding to the [`Serializer`] compound types: [`SerializeSeq`],
    /// [`SerializeTuple`], [`SerializeTupleStruct`], [`SerializeTupleVariant`],
    /// [`SerializeMap`], [`SerializeStruct`], and [`SerializeStructVariant`].
    ///
    /// ```edition2018
    /// # use serde::ser::{Serializer, Impossible};
    /// # use serde::__private::doc::Error;
    /// #
    /// # struct MySerializer;
    /// #
    /// impl Serializer for MySerializer {
    ///     type Ok = ();
    ///     type Error = Error;
    ///
    ///     type SerializeSeq = Impossible<(), Error>;
    ///     /* other associated types */
    ///
    ///     /// This data format does not support serializing sequences.
    ///     fn serialize_seq(self,
    ///                      len: Option<usize>)
    ///                      -> Result<Self::SerializeSeq, Error> {
    ///         // Given Impossible cannot be instantiated, the only
    ///         // thing we can do here is to return an error.
    /// #         stringify! {
    ///         Err(...)
    /// #         };
    /// #         unimplemented!()
    ///     }
    ///
    ///     /* other Serializer methods */
    /// #     serde::__serialize_unimplemented! {
    /// #         bool i8 i16 i32 i64 u8 u16 u32 u64 f32 f64 char str bytes none some
    /// #         unit unit_struct unit_variant newtype_struct newtype_variant
    /// #         tuple tuple_struct tuple_variant map struct struct_variant
    /// #     }
    /// }
    /// ```
    ///
    /// [`Serializer`]: trait.Serializer.html
    /// [`SerializeSeq`]: trait.SerializeSeq.html
    /// [`SerializeTuple`]: trait.SerializeTuple.html
    /// [`SerializeTupleStruct`]: trait.SerializeTupleStruct.html
    /// [`SerializeTupleVariant`]: trait.SerializeTupleVariant.html
    /// [`SerializeMap`]: trait.SerializeMap.html
    /// [`SerializeStruct`]: trait.SerializeStruct.html
    /// [`SerializeStructVariant`]: trait.SerializeStructVariant.html
    pub struct Impossible<Ok, Error> {
        void: Void,
        ok: PhantomData<Ok>,
        error: PhantomData<Error>,
    }

    enum Void {}

    impl<Ok, Error> SerializeSeq for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeTuple for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_element<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeTupleStruct for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeTupleVariant for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_field<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeMap for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_key<T: ?Sized>(&mut self, key: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = key;
            match self.void {}
        }

        fn serialize_value<T: ?Sized>(&mut self, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeStruct for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = key;
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }

    impl<Ok, Error> SerializeStructVariant for Impossible<Ok, Error>
    where
        Error: ser::Error,
    {
        type Ok = Ok;
        type Error = Error;

        fn serialize_field<T: ?Sized>(&mut self, key: &'static str, value: &T) -> Result<(), Error>
        where
            T: Serialize,
        {
            let _ = key;
            let _ = value;
            match self.void {}
        }

        fn end(self) -> Result<Ok, Error> {
            match self.void {}
        }
    }