diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index ef2a33c37dd30..6da1a3da5fd60 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -65,6 +65,7 @@
 #![allow(raw_pointer_derive)]
 #![deny(missing_docs)]
 
+#![feature(associated_type_defaults)]
 #![feature(intrinsics)]
 #![feature(lang_items)]
 #![feature(on_unimplemented)]
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 53b57cdfaa1ad..945e457a77b73 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -163,8 +163,12 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Status)] = &[
 
     // Allows the definition recursive static items.
     ("static_recursion", "1.3.0", Active),
-// Allows default type parameters to influence type inference.
-    ("default_type_parameter_fallback", "1.3.0", Active)
+
+    // Allows default type parameters to influence type inference.
+    ("default_type_parameter_fallback", "1.3.0", Active),
+
+    // Allows associated type defaults
+    ("associated_type_defaults", "1.2.0", Active),
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -762,6 +766,10 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                     self.gate_feature("const_fn", ti.span, "const fn is unstable");
                 }
             }
+            ast::TypeTraitItem(_, Some(_)) => {
+                self.gate_feature("associated_type_defaults", ti.span,
+                                  "associated type defaults are unstable");
+            }
             _ => {}
         }
         visit::walk_trait_item(self, ti);
diff --git a/src/test/auxiliary/xcrate_associated_type_defaults.rs b/src/test/auxiliary/xcrate_associated_type_defaults.rs
index a6b70bf974f95..43852a4e793f3 100644
--- a/src/test/auxiliary/xcrate_associated_type_defaults.rs
+++ b/src/test/auxiliary/xcrate_associated_type_defaults.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 pub trait Foo {
     type Input = usize;
     fn bar(&self, _: Self::Input) {}
diff --git a/src/test/compile-fail/associated-types-overridden-default.rs b/src/test/compile-fail/associated-types-overridden-default.rs
index eb519e79006d2..19f13f5fc2f2b 100644
--- a/src/test/compile-fail/associated-types-overridden-default.rs
+++ b/src/test/compile-fail/associated-types-overridden-default.rs
@@ -9,6 +9,7 @@
 // except according to those terms.
 
 #![feature(associated_consts)]
+#![feature(associated_type_defaults)]
 
 pub trait Tr {
     type Assoc = u8;
diff --git a/src/test/compile-fail/feature-gate-assoc-type-defaults.rs b/src/test/compile-fail/feature-gate-assoc-type-defaults.rs
new file mode 100644
index 0000000000000..fc4871a712dff
--- /dev/null
+++ b/src/test/compile-fail/feature-gate-assoc-type-defaults.rs
@@ -0,0 +1,15 @@
+// Copyright 2015 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 Foo {
+    type Bar = u8; //~ ERROR associated type defaults are unstable
+}
+
+fn main() {}
diff --git a/src/test/compile-fail/issue-23073.rs b/src/test/compile-fail/issue-23073.rs
index 1286ba873be5a..2d219177a80d4 100644
--- a/src/test/compile-fail/issue-23073.rs
+++ b/src/test/compile-fail/issue-23073.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 trait Foo { type T; }
 trait Bar {
     type Foo: Foo;
diff --git a/src/test/compile-fail/issue-23595-1.rs b/src/test/compile-fail/issue-23595-1.rs
index 749b261e38719..a3422d859c61f 100644
--- a/src/test/compile-fail/issue-23595-1.rs
+++ b/src/test/compile-fail/issue-23595-1.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 use std::ops::{Index};
 
 trait Hierarchy {
diff --git a/src/test/compile-fail/issue-23595-2.rs b/src/test/compile-fail/issue-23595-2.rs
index 78a3f42f1a6a7..6a3ce03fce5e0 100644
--- a/src/test/compile-fail/issue-23595-2.rs
+++ b/src/test/compile-fail/issue-23595-2.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 pub struct C<AType: A> {a:AType}
 
 pub trait A {
diff --git a/src/test/compile-fail/lint-missing-doc.rs b/src/test/compile-fail/lint-missing-doc.rs
index 04db6c8c8f39c..c98d7083743b8 100644
--- a/src/test/compile-fail/lint-missing-doc.rs
+++ b/src/test/compile-fail/lint-missing-doc.rs
@@ -12,6 +12,7 @@
 // injected intrinsics by the compiler.
 #![deny(missing_docs)]
 #![allow(dead_code)]
+#![feature(associated_type_defaults)]
 
 //! Some garbage docs for the crate here
 #![doc="More garbage"]
diff --git a/src/test/run-pass/default-associated-types.rs b/src/test/run-pass/default-associated-types.rs
index b3def429b9b8c..3e6c72c993a0a 100644
--- a/src/test/run-pass/default-associated-types.rs
+++ b/src/test/run-pass/default-associated-types.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 trait Foo<T> {
     type Out = T;
     fn foo(&self) -> Self::Out;
diff --git a/src/test/run-pass/issue-25339.rs b/src/test/run-pass/issue-25339.rs
index af172000fdb1a..381df7c5d5938 100644
--- a/src/test/run-pass/issue-25339.rs
+++ b/src/test/run-pass/issue-25339.rs
@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(associated_type_defaults)]
+
 use std::marker::PhantomData;
 
 pub trait Routing<I> {