Allow negative impls just for Send and Sync

src/librustc_typeck/check/wf.rs

@@ -76,6 +76,18 @@ impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
             ast::ItemImpl(_, ast::ImplPolarity::Positive, _, _, _, _) => {
                 self.check_impl(item);
             }
+            ast::ItemImpl(_, ast::ImplPolarity::Negative, _, Some(ref tref), _, _) => {
+                let trait_ref = ty::node_id_to_trait_ref(ccx.tcx, tref.ref_id);
+                match ccx.tcx.lang_items.to_builtin_kind(trait_ref.def_id) {
+                    Some(ty::BoundSend) | Some(ty::BoundSync) => {}
+                    Some(_) | None => {
+                        ccx.tcx.sess.span_err(
+                            item.span,
+                            format!("negative impls are currently \
+                                     allowed just for `Send` and `Sync`").as_slice())
+                    }
+                }
+            }
             ast::ItemFn(..) => {
                 self.check_item_type(item);
             }

src/test/compile-fail/typeck-negative-impls-builtin.rs

@@ -0,0 +1,20 @@
+// Copyright 2014 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.
+
+#![feature(optin_builtin_traits)]
+
+struct TestType;
+
+trait TestTrait {}
+
+impl !TestTrait for TestType {}
+//~^ ERROR  negative impls are currently allowed just for `Send` and `Sync`
+
+fn main() {}