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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 {}
+ }
+}