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diff --git a/third_party/rust/serde/src/ser/impossible.rs b/third_party/rust/serde/src/ser/impossible.rs
new file mode 100644
index 0000000000..e8df9ca7aa
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+++ b/third_party/rust/serde/src/ser/impossible.rs
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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 {}
+    }
+}