error: expected serde rename attribute to be a string: `rename = "..."`
 --> tests/ui/expected-string/character.rs:5:22
  |
3 | #[derive(Serialize)]
  |          --------- in this derive macro expansion
4 | struct S {
5 |     #[serde(rename = 'a')]
  |                      ^^^
  |
  = note: this error originates in the derive macro `Serialize` (in Nightly builds, run with -Z macro-backtrace for more info)