error: malformed rename attribute, expected `rename(serialize = ..., deserialize = ...)`
 --> tests/ui/malformed/rename.rs:5:20
  |
3 | #[derive(Serialize)]
  |          --------- in this derive macro expansion
4 | struct S {
5 |     #[serde(rename(unknown))]
  |                    ^^^^^^^
  |
  = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)