Only implement Fn* traits for extern "Rust" safe function pointers.

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

@@ -291,6 +291,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             return;
         }
 
+        // Keep this funtion in sync with extract_tupled_inputs_and_output_from_callable
+        // until the old solver (and thus this function) is removed.
+
         // Okay to skip binder because what we are inspecting doesn't involve bound regions.
         let self_ty = obligation.self_ty().skip_binder();
         match *self_ty.kind() {