Disable feature(unboxed_closures, fn_traits) in weird-exprs

One shouldn't need a nightly compiler in order to ~~have fun~~ call a
function many times.

tests/ui/weird-exprs.rs

@@ -1,7 +1,6 @@
 // run-pass
 
 #![feature(generators)]
-#![feature(unboxed_closures, fn_traits)]
 
 #![allow(non_camel_case_types)]
 #![allow(dead_code)]
@@ -17,6 +16,7 @@
 extern crate core;
 use std::cell::Cell;
 use std::mem::swap;
+use std::ops::Deref;
 
 // Just a grab bag of stuff that you wouldn't want to actually write.
 
@@ -183,10 +183,10 @@ fn 𝚌𝚘𝚗𝚝𝚒𝚗𝚞𝚎() {
 
 fn function() {
     struct foo;
-    impl FnOnce<()> for foo {
-        type Output = foo;
-        extern "rust-call" fn call_once(self, _args: ()) -> Self::Output {
-            foo
+    impl Deref for foo {
+        type Target = fn() -> Self;
+        fn deref(&self) -> &Self::Target {
+            &((|| foo) as _)
         }
     }
     let foo = foo () ()() ()()() ()()()() ()()()()();