Future incompatibility report: Future breakage diagnostic:
warning: to use a constant of type `Cow<'_, str>` in a pattern, `Cow<'_, str>` must be annotated with `#[derive(PartialEq)]`
  --> $DIR/issue-89088.rs:19:9
   |
LL |         FOO => todo!(),
   |         ^^^
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #120362 <https://github.com/rust-lang/rust/issues/120362>
   = note: the traits must be derived, manual `impl`s are not sufficient
   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details