error: #[serde(flatten)] cannot be used on tuple structs
 --> tests/ui/conflict/flatten-tuple-struct.rs:6:17
  |
5 | #[derive(Serialize)]
  |          --------- in this derive macro expansion
6 | struct Foo(u32, #[serde(flatten)] HashMap<String, String>);
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)