Auto merge of #118120 - compiler-errors:closure-kind, r=lcnr

Remove `PredicateKind::ClosureKind`

We don't need the `ClosureKind` predicate kind -- instead, `Fn`-family trait goals are left as ambiguous, and we only need to make progress on `FnOnce` projection goals for inference purposes.

This is similar to how we do confirmation of `Fn`-family trait and projection goals in the new trait solver, which also doesn't use the `ClosureKind` predicate.

Some hacky logic is added in the second commit so that we can keep the error messages the same.

compiler/rustc_trait_selection/src/solve/eval_ctxt/mod.rs

@@ -406,8 +406,6 @@ impl<'a, 'tcx> EvalCtxt<'a, 'tcx> {
                 ty::PredicateKind::Coerce(predicate) => {
                     self.compute_coerce_goal(Goal { param_env, predicate })
                 }
-                ty::PredicateKind::ClosureKind(def_id, args, kind) => self
-                    .compute_closure_kind_goal(Goal { param_env, predicate: (def_id, args, kind) }),
                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
                     self.compute_object_safe_goal(trait_def_id)
                 }

compiler/rustc_trait_selection/src/solve/fulfill.rs

@@ -135,7 +135,6 @@ impl<'tcx> TraitEngine<'tcx> for FulfillmentCtxt<'tcx> {
                                     }
                                     ty::PredicateKind::Clause(_)
                                     | ty::PredicateKind::ObjectSafe(_)
-                                    | ty::PredicateKind::ClosureKind(_, _, _)
                                     | ty::PredicateKind::Ambiguous => {
                                         FulfillmentErrorCode::CodeSelectionError(
                                             SelectionError::Unimplemented,