Don't allow default impls for traits outside their crate

src/librustc_typeck/coherence/orphan.rs

@@ -96,6 +96,16 @@ fn visit_item(&mut self, item: &'v ast::Item) {
                     }
                 }
             }
+            ast::ItemDefTrait(_, ref ast_trait_ref) => {
+                // "Trait" impl
+                debug!("coherence2::orphan check: default trait impl {}", item.repr(self.tcx));
+                let trait_ref = ty::node_id_to_trait_ref(self.tcx, ast_trait_ref.ref_id);
+                if trait_ref.def_id.krate != ast::LOCAL_CRATE {
+                    span_err!(self.tcx.sess, item.span, E0316,
+                              "cannot create default implementations for traits outside the \
+                               crate they're defined in; define a new trait instead.");
+                }
+            }
             _ => {
                 // Not an impl
             }

src/librustc_typeck/diagnostics.rs

@@ -171,7 +171,8 @@
     E0247, // found module name used as a type
     E0248, // found value name used as a type
     E0249, // expected constant expr for array length
-    E0250  // expected constant expr for array length
+    E0250, // expected constant expr for array length
+    E0316  // can't create default impls for traits outside their crates
 }
 
 __build_diagnostic_array! { DIAGNOSTICS }