From 26186bddd1e4c194d144d9684d5189ce5f3865a4 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 10 Aug 2020 15:06:34 -0700
Subject: [PATCH 1/3] Add test for flattened unit

---
 test_suite/tests/test_annotations.rs | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/test_suite/tests/test_annotations.rs b/test_suite/tests/test_annotations.rs
index 06c485df..7604fe03 100644
--- a/test_suite/tests/test_annotations.rs
+++ b/test_suite/tests/test_annotations.rs
@@ -1967,6 +1967,29 @@ fn test_flatten_map_twice() {
     );
 }
 
+#[test]
+fn test_flatten_unit() {
+    #[derive(Debug, PartialEq, Serialize, Deserialize)]
+    struct Response<T> {
+        #[serde(flatten)]
+        data: T,
+        status: usize,
+    }
+
+    assert_tokens(
+        &Response {
+            data: (),
+            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)]

From ba0707559026b7dacef46de044a0da54ef64d50a Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 10 Aug 2020 15:06:47 -0700
Subject: [PATCH 2/3] Support serializing flattened unit

---
 serde/src/private/ser.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/serde/src/private/ser.rs b/serde/src/private/ser.rs
index 70f33818..eb8cfc98 100644
--- a/serde/src/private/ser.rs
+++ b/serde/src/private/ser.rs
@@ -1124,7 +1124,7 @@ where
     }
 
     fn serialize_unit(self) -> Result<Self::Ok, Self::Error> {
-        Err(Self::bad_type(Unsupported::Unit))
+        Ok(())
     }
 
     fn serialize_unit_struct(self, _: &'static str) -> Result<Self::Ok, Self::Error> {

From bf76f502942b31da3e755c2775128d88edac27d1 Mon Sep 17 00:00:00 2001
From: David Tolnay <dtolnay@gmail.com>
Date: Mon, 10 Aug 2020 15:06:52 -0700
Subject: [PATCH 3/3] Support deserializing flattened unit

---
 serde/src/private/de.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/serde/src/private/de.rs b/serde/src/private/de.rs
index eb5424cc..bcb964a9 100644
--- a/serde/src/private/de.rs
+++ b/serde/src/private/de.rs
@@ -2763,6 +2763,13 @@ where
         }
     }
 
+    fn deserialize_unit<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_unit()
+    }
+
     forward_to_deserialize_other! {
         deserialize_bool()
         deserialize_i8()
@@ -2780,7 +2787,6 @@ where
         deserialize_string()
         deserialize_bytes()
         deserialize_byte_buf()
-        deserialize_unit()
         deserialize_unit_struct(&'static str)
         deserialize_seq()
         deserialize_tuple(usize)