Remove serialize_unit from Formatter's impl

serde/src/ser/fmt.rs

@@ -45,10 +45,6 @@ impl<'a, 'b> Serializer for &'a mut fmt::Formatter<'b> {
         serialize_unit_struct: &'static str,
     }
 
-    fn serialize_unit(self) -> fmt::Result {
-        Ok(())
-    }
-
     fn serialize_unit_variant(
         self,
         _name: &'static str,
@@ -73,6 +69,10 @@ impl<'a, 'b> Serializer for &'a mut fmt::Formatter<'b> {
         Err(fmt::Error)
     }
 
+    fn serialize_unit(self) -> fmt::Result {
+        Err(fmt::Error)
+    }
+
     fn serialize_newtype_struct<T: ?Sized>(self, _name: &'static str, _value: &T) -> fmt::Result
     where
         T: Serialize,