error: #[serde(borrow)] may only be used on newtype variants
 --> tests/ui/borrow/struct_variant.rs:8:5
  |
6 |   #[derive(Deserialize)]
  |            ----------- in this derive macro expansion
7 |   enum Test<'a> {
8 | /     #[serde(borrow)]
9 | |     S { s: Str<'a> },
  | |____________________^
  |
  = note: this error originates in the derive macro `Deserialize` (in Nightly builds, run with -Z macro-backtrace for more info)