Avoid ever constructing cyclic types in the first place, rather than detecting them in resolve. This simplifies logic elsewhere in the compiler. cc #5527

src/test/compile-fail/occurs-check-2.rs

@@ -0,0 +1,18 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use std::gc::GC;
+
+fn main() {
+    let f;
+    let g;
+    g = f;
+    f = box(GC) g; //~ ERROR cyclic type of infinite size
+}

src/test/compile-fail/occurs-check.rs

@@ -12,6 +12,6 @@
 use std::gc::GC;
 
 fn main() {
-    let f; //~ ERROR cyclic type of infinite size
-    f = box(GC) f;
+    let f;
+    f = box(GC) f; //~ ERROR cyclic type of infinite size
 }