Add TyCtxt::is_fn_trait

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -489,7 +489,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         // We provide impl of all fn traits for fn pointers.
-        if self.tcx().fn_trait_kind_from_def_id(obligation.predicate.def_id()).is_none() {
+        if !self.tcx().is_fn_trait(obligation.predicate.def_id()) {
             return;
         }