Allow to flatten IgnoredAny to ignore any additional data

Although any additional fields in struct by default are ignored, sometimes
this can be useful, if you use generic structures, for example

serde/src/private/de.rs

@@ -2811,6 +2811,13 @@ where
         visitor.visit_unit()
     }
 
+    fn deserialize_ignored_any<V>(self, visitor: V) -> Result<V::Value, Self::Error>
+    where
+        V: Visitor<'de>,
+    {
+        visitor.visit_unit()
+    }
+
     forward_to_deserialize_other! {
         deserialize_bool()
         deserialize_i8()
@@ -2833,7 +2840,6 @@ where
         deserialize_tuple(usize)
         deserialize_tuple_struct(&'static str, usize)
         deserialize_identifier()
-        deserialize_ignored_any()
     }
 }
 

test_suite/tests/test_annotations.rs

@@ -10,7 +10,7 @@
     clippy::uninlined_format_args,
 )]
 
-use serde::de::{self, MapAccess, Unexpected, Visitor};
+use serde::de::{self, IgnoredAny, MapAccess, Unexpected, Visitor};
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 
 use std::collections::{BTreeMap, HashMap};
@@ -2494,6 +2494,31 @@ fn test_flatten_option() {
     );
 }
 
+#[test]
+fn test_flatten_ignored_any() {
+    #[derive(Deserialize, PartialEq, Debug)]
+    struct Outer {
+        #[serde(flatten)]
+        inner: IgnoredAny,
+    }
+
+    assert_de_tokens(
+        &Outer { inner: IgnoredAny },
+        &[Token::Map { len: None }, Token::MapEnd],
+    );
+
+    assert_de_tokens(
+        &Outer { inner: IgnoredAny },
+        &[
+            Token::Struct {
+                name: "DoNotMatter",
+                len: 0,
+            },
+            Token::StructEnd,
+        ],
+    );
+}
+
 #[test]
 fn test_transparent_struct() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]