diff --git a/src/libcore/marker.rs b/src/libcore/marker.rs
index 1b866501b8ea1..0197ee82c7740 100644
--- a/src/libcore/marker.rs
+++ b/src/libcore/marker.rs
@@ -39,6 +39,8 @@ pub unsafe trait Send : MarkerTrait {
     // empty.
 }
 
+unsafe impl Send for .. { }
+
 impl<T> !Send for *const T { }
 impl<T> !Send for *mut T { }
 impl !Send for Managed { }
@@ -203,6 +205,8 @@ pub unsafe trait Sync : MarkerTrait {
     // Empty
 }
 
+unsafe impl Sync for .. { }
+
 impl<T> !Sync for *const T { }
 impl<T> !Sync for *mut T { }
 impl !Sync for Managed { }
diff --git a/src/librustc/middle/ty.rs b/src/librustc/middle/ty.rs
index 99c35c6e54258..c4fcf882cfb03 100644
--- a/src/librustc/middle/ty.rs
+++ b/src/librustc/middle/ty.rs
@@ -5980,10 +5980,7 @@ pub fn item_variances(tcx: &ctxt, item_id: ast::DefId) -> Rc<ItemVariances> {
 
 pub fn trait_has_default_impl(tcx: &ctxt, trait_def_id: DefId) -> bool {
     populate_implementations_for_trait_if_necessary(tcx, trait_def_id);
-    match tcx.lang_items.to_builtin_kind(trait_def_id) {
-        Some(BoundSend) | Some(BoundSync) => true,
-        _ => tcx.traits_with_default_impls.borrow().contains_key(&trait_def_id),
-    }
+    tcx.traits_with_default_impls.borrow().contains_key(&trait_def_id)
 }
 
 /// Records a trait-to-implementation mapping.
diff --git a/src/librustc_typeck/coherence/unsafety.rs b/src/librustc_typeck/coherence/unsafety.rs
index 867dea9588568..e4926b119d5db 100644
--- a/src/librustc_typeck/coherence/unsafety.rs
+++ b/src/librustc_typeck/coherence/unsafety.rs
@@ -27,55 +27,66 @@ struct UnsafetyChecker<'cx, 'tcx:'cx> {
     tcx: &'cx ty::ctxt<'tcx>
 }
 
-impl<'cx, 'tcx,'v> visit::Visitor<'v> for UnsafetyChecker<'cx, 'tcx> {
-    fn visit_item(&mut self, item: &'v ast::Item) {
-        match item.node {
-            ast::ItemImpl(unsafety, polarity, _, _, _, _) => {
-                match ty::impl_trait_ref(self.tcx, ast_util::local_def(item.id)) {
-                    None => {
-                        // Inherent impl.
-                        match unsafety {
-                            ast::Unsafety::Normal => { /* OK */ }
-                            ast::Unsafety::Unsafe => {
-                                span_err!(self.tcx.sess, item.span, E0197,
-                                    "inherent impls cannot be declared as unsafe");
-                            }
-                        }
+impl<'cx, 'tcx, 'v> UnsafetyChecker<'cx, 'tcx> {
+    fn check_unsafety_coherence(&mut self, item: &'v ast::Item,
+                                unsafety: ast::Unsafety,
+                                polarity: ast::ImplPolarity) {
+        match ty::impl_trait_ref(self.tcx, ast_util::local_def(item.id)) {
+            None => {
+                // Inherent impl.
+                match unsafety {
+                    ast::Unsafety::Normal => { /* OK */ }
+                    ast::Unsafety::Unsafe => {
+                        span_err!(self.tcx.sess, item.span, E0197,
+                                  "inherent impls cannot be declared as unsafe");
                     }
+                }
+            }
 
-                    Some(trait_ref) => {
-                        let trait_def = ty::lookup_trait_def(self.tcx, trait_ref.def_id);
-                        match (trait_def.unsafety, unsafety, polarity) {
-                            (ast::Unsafety::Unsafe,
-                             ast::Unsafety::Unsafe, ast::ImplPolarity::Negative) => {
-                                span_err!(self.tcx.sess, item.span, E0198,
-                                    "negative implementations are not unsafe");
-                            }
+            Some(trait_ref) => {
+                let trait_def = ty::lookup_trait_def(self.tcx, trait_ref.def_id);
+                match (trait_def.unsafety, unsafety, polarity) {
+                    (ast::Unsafety::Unsafe,
+                     ast::Unsafety::Unsafe, ast::ImplPolarity::Negative) => {
+                        span_err!(self.tcx.sess, item.span, E0198,
+                                  "negative implementations are not unsafe");
+                    }
 
-                            (ast::Unsafety::Normal, ast::Unsafety::Unsafe, _) => {
-                                span_err!(self.tcx.sess, item.span, E0199,
-                                    "implementing the trait `{}` is not unsafe",
-                                            trait_ref.user_string(self.tcx));
-                            }
+                    (ast::Unsafety::Normal, ast::Unsafety::Unsafe, _) => {
+                        span_err!(self.tcx.sess, item.span, E0199,
+                                  "implementing the trait `{}` is not unsafe",
+                                  trait_ref.user_string(self.tcx));
+                    }
 
-                            (ast::Unsafety::Unsafe,
-                             ast::Unsafety::Normal, ast::ImplPolarity::Positive) => {
-                                span_err!(self.tcx.sess, item.span, E0200,
-                                    "the trait `{}` requires an `unsafe impl` declaration",
-                                            trait_ref.user_string(self.tcx));
-                            }
+                    (ast::Unsafety::Unsafe,
+                     ast::Unsafety::Normal, ast::ImplPolarity::Positive) => {
+                        span_err!(self.tcx.sess, item.span, E0200,
+                                  "the trait `{}` requires an `unsafe impl` declaration",
+                                  trait_ref.user_string(self.tcx));
+                    }
 
-                            (ast::Unsafety::Unsafe,
-                             ast::Unsafety::Normal, ast::ImplPolarity::Negative) |
-                            (ast::Unsafety::Unsafe,
-                             ast::Unsafety::Unsafe, ast::ImplPolarity::Positive) |
-                            (ast::Unsafety::Normal, ast::Unsafety::Normal, _) => {
-                                /* OK */
-                            }
-                        }
+                    (ast::Unsafety::Unsafe,
+                     ast::Unsafety::Normal, ast::ImplPolarity::Negative) |
+                    (ast::Unsafety::Unsafe,
+                     ast::Unsafety::Unsafe, ast::ImplPolarity::Positive) |
+                    (ast::Unsafety::Normal, ast::Unsafety::Normal, _) => {
+                        /* OK */
                     }
                 }
             }
+        }
+    }
+}
+
+impl<'cx, 'tcx,'v> visit::Visitor<'v> for UnsafetyChecker<'cx, 'tcx> {
+    fn visit_item(&mut self, item: &'v ast::Item) {
+        match item.node {
+            ast::ItemDefaultImpl(unsafety, _) => {
+                self.check_unsafety_coherence(item, unsafety, ast::ImplPolarity::Positive);
+            }
+            ast::ItemImpl(unsafety, polarity, _, _, _, _) => {
+                self.check_unsafety_coherence(item, unsafety, polarity);
+            }
             _ => { }
         }
 
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 0a9980c892527..f5e891a9dd643 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -563,6 +563,13 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                 }
             }
 
+            ast::ItemDefaultImpl(..) => {
+                self.gate_feature("optin_builtin_traits",
+                                  i.span,
+                                  "default trait implementations are experimental \
+                                   and possibly buggy");
+            }
+
             ast::ItemImpl(_, polarity, _, _, _, _) => {
                 match polarity {
                     ast::ImplPolarity::Negative => {
diff --git a/src/test/compile-fail/coherence-default-trait-impl.rs b/src/test/compile-fail/coherence-default-trait-impl.rs
index 6bcbefb904d9e..a5b317307379e 100644
--- a/src/test/compile-fail/coherence-default-trait-impl.rs
+++ b/src/test/compile-fail/coherence-default-trait-impl.rs
@@ -21,4 +21,14 @@ impl MyTrait for .. {}
 impl MyTrait for .. {}
 //~^ ERROR conflicting implementations for trait `MyTrait`
 
+trait MySafeTrait: MarkerTrait {}
+
+unsafe impl MySafeTrait for .. {}
+//~^ ERROR implementing the trait `MySafeTrait` is not unsafe
+
+unsafe trait MyUnsafeTrait: MarkerTrait {}
+
+impl MyUnsafeTrait for .. {}
+//~^ ERROR the trait `MyUnsafeTrait` requires an `unsafe impl` declaration
+
 fn main() {}
diff --git a/src/test/run-make/rustdoc-default-impl/foo.rs b/src/test/run-make/rustdoc-default-impl/foo.rs
index 08f3bd10e74a7..4c30c7b22a60d 100644
--- a/src/test/run-make/rustdoc-default-impl/foo.rs
+++ b/src/test/run-make/rustdoc-default-impl/foo.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(optin_builtin_traits)]
+
 pub mod bar {
     use std::marker;