Add regression test for issue #1904

Currently panics in derive:

error: proc-macro derive panicked
    --> test_suite\tests\test_annotations.rs:2386:25
     |
2386 |     #[derive(Serialize, Deserialize, PartialEq, Debug)]
     |                         ^^^^^^^^^^^
     |
     = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
  --> test_suite\tests\regression\issue1904.rs:57:10
   |
57 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
  --> test_suite\tests\regression\issue1904.rs:47:10
   |
47 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
  --> test_suite\tests\regression\issue1904.rs:37:10
   |
37 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
  --> test_suite\tests\regression\issue1904.rs:27:10
   |
27 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
  --> test_suite\tests\regression\issue1904.rs:16:10
   |
16 | #[derive(Deserialize)]
   |          ^^^^^^^^^^^
   |
   = help: message: assertion failed: !cattrs.has_flatten()

error: proc-macro derive panicked
 --> test_suite\tests\regression\issue1904.rs:7:10
  |
7 | #[derive(Deserialize)]
  |          ^^^^^^^^^^^
  |
  = help: message: assertion failed: !cattrs.has_flatten()

test_suite/tests/test_annotations.rs

@@ -2380,6 +2380,56 @@ fn test_partially_untagged_enum_desugared() {
     );
 }
 
+/// Regression test for https://github.com/serde-rs/serde/issues/1904
+#[test]
+fn test_enum_tuple_and_struct_with_flatten() {
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    enum Outer {
+        Tuple(f64, i32),
+        Flatten {
+            #[serde(flatten)]
+            nested: Nested,
+        },
+    }
+
+    #[derive(Serialize, Deserialize, PartialEq, Debug)]
+    struct Nested {
+        a: i32,
+        b: i32,
+    }
+
+    assert_tokens(
+        &Outer::Tuple(1.2, 3),
+        &[
+            Token::TupleVariant {
+                name: "Outer",
+                variant: "Tuple",
+                len: 2,
+            },
+            Token::F64(1.2),
+            Token::I32(3),
+            Token::TupleVariantEnd,
+        ],
+    );
+    assert_tokens(
+        &Outer::Flatten {
+            nested: Nested { a: 1, b: 2 },
+        },
+        &[
+            Token::NewtypeVariant {
+                name: "Outer",
+                variant: "Flatten",
+            },
+            Token::Map { len: None },
+            Token::Str("a"),
+            Token::I32(1),
+            Token::Str("b"),
+            Token::I32(2),
+            Token::MapEnd,
+        ],
+    );
+}
+
 #[test]
 fn test_partially_untagged_internally_tagged_enum() {
     #[derive(Serialize, Deserialize, PartialEq, Debug)]