From 2dddc7796d436b2bfa0d1016d252250a3f7c4d18 Mon Sep 17 00:00:00 2001
From: Mingun <alexander_sergey@mail.ru>
Date: Sat, 17 Aug 2024 11:58:28 +0500
Subject: [PATCH] Cover ContentRefDeserializer::deserialize_option

---
 serde/src/private/de.rs                |  7 +++
 test_suite/tests/test_enum_untagged.rs | 74 ++++++++++++++++++++++++++
 2 files changed, 81 insertions(+)

diff --git a/serde/src/private/de.rs b/serde/src/private/de.rs
index 6c81b720..5b5d9b6a 100644
--- a/serde/src/private/de.rs
+++ b/serde/src/private/de.rs
@@ -1898,10 +1898,17 @@ mod content {
         where
             V: Visitor<'de>,
         {
+            // Covered by tests/test_enum_untagged.rs
+            //      with_optional_field::*
             match *self.content {
                 Content::None => visitor.visit_none(),
                 Content::Some(ref v) => visitor.visit_some(ContentRefDeserializer::new(v)),
                 Content::Unit => visitor.visit_unit(),
+                // This case is necessary for formats which does not store marker of optionality of value,
+                // for example, JSON. When `deserialize_any` is requested from such formats, they will
+                // report value without using `Visitor::visit_some`, because they do not known in which
+                // contexts this value will be used.
+                // RON is example of format which preserve markers.
                 _ => visitor.visit_some(self),
             }
         }
diff --git a/test_suite/tests/test_enum_untagged.rs b/test_suite/tests/test_enum_untagged.rs
index d06002ce..60c62bac 100644
--- a/test_suite/tests/test_enum_untagged.rs
+++ b/test_suite/tests/test_enum_untagged.rs
@@ -191,6 +191,80 @@ fn newtype_enum() {
     );
 }
 
+// Reaches crate::private::de::content::ContentRefDeserializer::deserialize_option
+mod with_optional_field {
+    use super::*;
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    #[serde(untagged)]
+    enum Enum {
+        Struct { optional: Option<u32> },
+        Null,
+    }
+
+    #[test]
+    fn some() {
+        assert_tokens(
+            &Enum::Struct { optional: Some(42) },
+            &[
+                Token::Struct {
+                    name: "Enum",
+                    len: 1,
+                },
+                Token::Str("optional"),
+                Token::Some,
+                Token::U32(42),
+                Token::StructEnd,
+            ],
+        );
+    }
+
+    #[test]
+    fn some_without_marker() {
+        assert_de_tokens(
+            &Enum::Struct { optional: Some(42) },
+            &[
+                Token::Struct {
+                    name: "Enum",
+                    len: 1,
+                },
+                Token::Str("optional"),
+                Token::U32(42),
+                Token::StructEnd,
+            ],
+        );
+    }
+
+    #[test]
+    fn none() {
+        assert_tokens(
+            &Enum::Struct { optional: None },
+            &[
+                Token::Struct {
+                    name: "Enum",
+                    len: 1,
+                },
+                Token::Str("optional"),
+                Token::None,
+                Token::StructEnd,
+            ],
+        );
+    }
+
+    #[test]
+    fn unit() {
+        assert_de_tokens(
+            &Enum::Struct { optional: None },
+            &[
+                Token::Map { len: None },
+                Token::Str("optional"),
+                Token::Unit,
+                Token::MapEnd,
+            ],
+        );
+    }
+}
+
 #[test]
 fn string_and_bytes() {
     #[derive(Debug, PartialEq, Deserialize)]