error: to use a constant of type `Foo` in a pattern, `Foo` must be annotated with `#[derive(PartialEq, Eq)]`
  --> $DIR/ice-6254.rs:13:9
   |
LL |         FOO_REF_REF => {},
   |         ^^^^^^^^^^^
   |
   = 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 #62411 <https://github.com/rust-lang/rust/issues/62411>
   = note: the traits must be derived, manual `impl`s are not sufficient
   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
   = note: `-D indirect-structural-match` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(indirect_structural_match)]`

error: aborting due to previous error