Complete coverage of ContentRefDeserializer::deserialize_newtype_struct

serde/src/private/de.rs

@@ -1938,10 +1938,17 @@ mod content {
         where
             V: Visitor<'de>,
         {
+            // Covered by tests/test_enum_untagged.rs
+            //      newtype_struct
             match *self.content {
                 Content::Newtype(ref v) => {
                     visitor.visit_newtype_struct(ContentRefDeserializer::new(v))
                 }
+                // This case is necessary for formats which does not store marker of a newtype,
+                // for example, JSON. When `deserialize_any` is requested from such formats, they will
+                // report value without using `Visitor::visit_newtype_struct`, because they do not
+                // known in which contexts this value will be used.
+                // RON is example of format which preserve markers.
                 _ => visitor.visit_newtype_struct(self),
             }
         }

test_suite/tests/test_enum_untagged.rs

@@ -104,6 +104,7 @@ fn newtype_unit_and_empty_map() {
     );
 }
 
+// Reaches crate::private::de::content::ContentRefDeserializer::deserialize_newtype_struct
 #[test]
 fn newtype_struct() {
     #[derive(Debug, PartialEq, Serialize, Deserialize)]
@@ -116,15 +117,21 @@ fn newtype_struct() {
         Null,
     }
 
+    let value = E::Newtype(NewtypeStruct(5));
+
+    // Content::Newtype case
     assert_tokens(
-        &E::Newtype(NewtypeStruct(5)),
+        &value,
         &[
             Token::NewtypeStruct {
                 name: "NewtypeStruct",
             },
             Token::U32(5),
         ],
     );
+
+    // _ case
+    assert_de_tokens(&value, &[Token::U32(5)]);
 }
 
 #[test]