Factor out JSON-specific Display impl for serde::de::Unexpected

src/error.rs

@@ -438,11 +438,11 @@ impl de::Error for Error {
 
     #[cold]
     fn invalid_type(unexp: de::Unexpected, exp: &dyn de::Expected) -> Self {
-        if let de::Unexpected::Unit = unexp {
-            Error::custom(format_args!("invalid type: null, expected {}", exp))
-        } else {
-            Error::custom(format_args!("invalid type: {}, expected {}", unexp, exp))
-        }
+        Error::custom(format_args!(
+            "invalid type: {}, expected {}",
+            JsonUnexpected(unexp),
+            exp,
+        ))
     }
 }
 
@@ -453,6 +453,18 @@ impl ser::Error for Error {
     }
 }
 
+struct JsonUnexpected<'a>(de::Unexpected<'a>);
+
+impl<'a> Display for JsonUnexpected<'a> {
+    fn fmt(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+        if let de::Unexpected::Unit = self.0 {
+            formatter.write_str("null")
+        } else {
+            Display::fmt(&self.0, formatter)
+        }
+    }
+}
+
 // Parse our own error message that looks like "{} at line {} column {}" to work
 // around erased-serde round-tripping the error through de::Error::custom.
 fn make_error(mut msg: String) -> Error {