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

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)]