From 4036ff88edc2ee85f288e8c9bc6d9a0f393cf96e Mon Sep 17 00:00:00 2001
From: Jon Gjengset <jon.gjengset@helsing.ai>
Date: Thu, 15 Aug 2024 15:50:40 +0200
Subject: [PATCH] Support (de-)serializing flattened unit struct

Fixes #2801.
---
 serde/src/private/de.rs              | 12 +++++++++++-
 serde/src/private/ser.rs             |  6 +-----
 test_suite/tests/test_annotations.rs | 26 ++++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 6 deletions(-)

diff --git a/serde/src/private/de.rs b/serde/src/private/de.rs
index c402d2c6..c146539e 100644
--- a/serde/src/private/de.rs
+++ b/serde/src/private/de.rs
@@ -2710,6 +2710,17 @@ where
         visitor.visit_unit()
     }
 
+    fn deserialize_unit_struct<V>(
+        self,
+        _name: &'static str,
+        visitor: V,
+    ) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_unit()
+    }
+
     fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
     where
         V: Visitor<'de>,
@@ -2734,7 +2745,6 @@ where
         deserialize_string()
         deserialize_bytes()
         deserialize_byte_buf()
-        deserialize_unit_struct(&'static str)
         deserialize_seq()
         deserialize_tuple(usize)
         deserialize_tuple_struct(&'static str, usize)
diff --git a/serde/src/private/ser.rs b/serde/src/private/ser.rs
index 9570629e..ebfeba97 100644
--- a/serde/src/private/ser.rs
+++ b/serde/src/private/ser.rs
@@ -51,8 +51,6 @@ enum Unsupported {
     String,
     ByteArray,
     Optional,
-    #[cfg(any(feature = "std", feature = "alloc"))]
-    UnitStruct,
     Sequence,
     Tuple,
     TupleStruct,
@@ -69,8 +67,6 @@ impl Display for Unsupported {
             Unsupported::String => formatter.write_str("a string"),
             Unsupported::ByteArray => formatter.write_str("a byte array"),
             Unsupported::Optional => formatter.write_str("an optional"),
-            #[cfg(any(feature = "std", feature = "alloc"))]
-            Unsupported::UnitStruct => formatter.write_str("unit struct"),
             Unsupported::Sequence => formatter.write_str("a sequence"),
             Unsupported::Tuple => formatter.write_str("a tuple"),
             Unsupported::TupleStruct => formatter.write_str("a tuple struct"),
@@ -1092,7 +1088,7 @@ where
     }
 
     fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {
-        Err(Self::bad_type(Unsupported::UnitStruct))
+        Ok(())
     }
 
     fn serialize_unit_variant(
diff --git a/test_suite/tests/test_annotations.rs b/test_suite/tests/test_annotations.rs
index 1174be6d..63cb3dd1 100644
--- a/test_suite/tests/test_annotations.rs
+++ b/test_suite/tests/test_annotations.rs
@@ -1815,6 +1815,32 @@ fn test_flatten_unit() {
     );
 }
 
+#[test]
+fn test_flatten_unit_struct() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Response<T> {
+        #[serde(flatten)]
+        data: T,
+        status: usize,
+    }
+
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Unit;
+
+    assert_tokens(
+        &Response {
+            data: Unit,
+            status: 0,
+        },
+        &[
+            Token::Map { len: None },
+            Token::Str("status"),
+            Token::U64(0),
+            Token::MapEnd,
+        ],
+    );
+}
+
 #[test]
 fn test_flatten_unsupported_type() {
     #[derive(Debug, PartialEq, Serialize, Deserialize)]