Macro to ignore tokens in doc test

serde/src/macros.rs

@@ -329,3 +329,10 @@ macro_rules! __serialize_unimplemented {
         )*
     };
 }
+
+/// Used only by Serde doc tests. Not public API.
+#[doc(hidden)]
+#[macro_export]
+macro_rules! __serde_ignore_tokens {
+    ($($tt:tt)+) => {}
+}

serde/src/ser/impossible.rs

@@ -35,9 +35,9 @@ use ser::{self, Serialize, SerializeSeq, SerializeTuple, SerializeTupleStruct,
 ///                      -> Result<Self::SerializeSeq, Error> {
 ///         // Given Impossible cannot be instantiated, the only
 ///         // thing we can do here is to return an error.
-/// #         macro_rules! ellipses { () => {
+/// #         __serde_ignore_tokens! {
 ///         Err(...)
-/// #         } }
+/// #         }
 /// #         unimplemented!()
 ///     }
 ///