Add a string argument to Error::syntax()

serde_tests/benches/bench_map.rs

@@ -17,7 +17,7 @@ pub enum Error {
 }
 
 impl serde::de::Error for Error {
-    fn syntax() -> Error { Error::SyntaxError }
+    fn syntax(_: &str) -> Error { Error::SyntaxError }
 
     fn end_of_stream() -> Error { Error::EndOfStream }