Forbid cross-polarity specializations

src/librustc/middle/traits/specialize/mod.rs

@@ -41,6 +41,7 @@ pub struct Overlap<'a, 'tcx: 'a> {
 /// Given a subst for the requested impl, translate it to a subst
 /// appropriate for the actual item definition (whether it be in that impl,
 /// a parent impl, or the trait).
+//
 // When we have selected one impl, but are actually using item definitions from
 // a parent impl providing a default, we need a way to translate between the
 // type parameters of the two impls. Here the `source_impl` is the one we've
@@ -153,6 +154,11 @@ pub fn specializes(tcx: &ty::ctxt, impl1_def_id: DefId, impl2_def_id: DefId) ->
     //
     // See RFC 1210 for more details and justification.
 
+    // Currently we do not allow e.g. a negative impl to specialize a positive one
+    if tcx.trait_impl_polarity(impl1_def_id) != tcx.trait_impl_polarity(impl2_def_id) {
+        return false
+    }
+
     let mut infcx = infer::normalizing_infer_ctxt(tcx, &tcx.tables);
 
     // Skiolemize impl1: we want to prove that "for all types matched by impl1,

src/test/compile-fail/specialization-polarity.rs

@@ -8,19 +8,23 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// Make sure specialization cannot change impl polarity
+
 #![feature(optin_builtin_traits)]
 #![feature(specialization)]
 
-struct TestType<T>(T);
+trait Foo {}
 
-// TODO: nail down the rules here with @nikomatsakis
+impl Foo for .. {}
 
-unsafe impl<T> Send for TestType<T> {}
-impl !Send for TestType<u8> {}
+impl<T> Foo for T {}
+impl !Foo for u8 {} //~ ERROR E0119
 
-fn assert_send<T: Send>() {}
+trait Bar {}
 
-fn main() {
-    assert_send::<TestType<()>>();
-    assert_send::<TestType<u8>>(); //~ ERROR
-}
+impl Bar for .. {}
+
+impl<T> !Bar for T {}
+impl Bar for u8 {} //~ ERROR E0119
+
+fn main() {}