forbid all self-referencing predicates

Fixes #38604

needs a crater run

Author: arielb1

src/librustc/traits/object_safety.rs

@@ -82,7 +82,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         let mut violations = vec![];
 
         for def_id in traits::supertrait_def_ids(self, trait_def_id) {
-            if self.supertraits_reference_self(def_id) {
+            if self.predicates_reference_self(def_id, true) {
                 violations.push(ObjectSafetyViolation::SupertraitSelf);
             }
         }
@@ -117,7 +117,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         if self.trait_has_sized_self(trait_def_id) {
             violations.push(ObjectSafetyViolation::SizedSelf);
         }
-        if self.supertraits_reference_self(trait_def_id) {
+        if self.predicates_reference_self(trait_def_id, false) {
             violations.push(ObjectSafetyViolation::SupertraitSelf);
         }
 
@@ -128,12 +128,20 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         violations
     }
 
-    fn supertraits_reference_self(self, trait_def_id: DefId) -> bool {
+    fn predicates_reference_self(
+        self,
+        trait_def_id: DefId,
+        supertraits_only: bool) -> bool
+    {
         let trait_ref = ty::Binder(ty::TraitRef {
             def_id: trait_def_id,
             substs: Substs::identity_for_item(self, trait_def_id)
         });
-        let predicates = self.item_super_predicates(trait_def_id);
+        let predicates = if supertraits_only {
+            self.item_super_predicates(trait_def_id)
+        } else {
+            self.item_predicates(trait_def_id)
+        };
         predicates
             .predicates
             .into_iter()

src/test/compile-fail/issue-38604.rs

@@ -0,0 +1,26 @@
+// Copyright 2016 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.
+
+trait Q<T:?Sized> {}
+trait Foo where u32: Q<Self> {
+    fn foo(&self);
+}
+
+impl Q<()> for u32 {}
+impl Foo for () {
+    fn foo(&self) {
+        println!("foo!");
+    }
+}
+
+fn main() {
+    let _f: Box<Foo> = //~ ERROR `Foo` cannot be made into an object
+        Box::new(()); //~ ERROR `Foo` cannot be made into an object
+}