allow enums that ref themselves via unsafe pointers

Author: nikomatsakis

src/rustc/middle/ty.rs

@@ -1475,11 +1475,14 @@ fn is_instantiable(cx: ctxt, r_ty: t) -> bool {
 
           ty_box(mt) |
           ty_uniq(mt) |
-          ty_ptr(mt) |
           ty_rptr(_, mt) {
             be type_requires(cx, seen, r_ty, mt.ty);
           }
 
+          ty_ptr(mt) {
+            false           // unsafe ptrs can always be NULL
+          }
+
           ty_rec(fields) {
             vec::any(fields) {|field|
                 type_requires(cx, seen, r_ty, field.mt.ty)