interpret: reduce usage of TypingEnv::fully_monomorphized

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -248,10 +248,9 @@ impl<'tcx> CompileTimeInterpCx<'tcx> {
         } else if self.tcx.is_lang_item(def_id, LangItem::PanicFmt) {
             // For panic_fmt, call const_panic_fmt instead.
             let const_def_id = self.tcx.require_lang_item(LangItem::ConstPanicFmt, None);
-            // FIXME(@lcnr): why does this use an empty env if we've got a `param_env` right here.
             let new_instance = ty::Instance::expect_resolve(
                 *self.tcx,
-                ty::TypingEnv::fully_monomorphized(),
+                self.typing_env(),
                 const_def_id,
                 instance.args,
                 self.cur_span(),