diff --git a/Cargo.toml b/Cargo.toml
index e3f8690..ebd53a7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -10,6 +10,7 @@ license = "MIT / Apache-2.0"
 [features]
 default = []
 plugins = ["serde", "serde_macros", "heapsize", "heapsize_plugin"]
+rustc_stable = []
 
 [dependencies]
 rustc-serialize = "0.3.2"
@@ -26,4 +27,3 @@ optional = true
 [dependencies.heapsize_plugin]
 version = "0.1.0"
 optional = true
-
diff --git a/src/length.rs b/src/length.rs
index 9676b55..0d9f4b2 100644
--- a/src/length.rs
+++ b/src/length.rs
@@ -9,13 +9,15 @@
 //! A one-dimensional length, tagged with its units.
 
 use scale_factor::ScaleFactor;
-use num::Zero;
+use num_lib::Zero;
 
 use num_lib::NumCast;
 #[cfg(feature = "plugins")]
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use std::cmp::Ordering;
-use std::ops::{Add, AddAssign, Sub, SubAssign, Mul, Div, Neg};
+use std::ops::{Add, Sub, Mul, Div, Neg};
+#[cfg(not(feature="rustc_stable"))]
+use std::ops::{AddAssign, SubAssign};
 use std::marker::PhantomData;
 
 /// A one-dimensional distance, with value represented by `T` and unit of measurement `Unit`.
@@ -71,6 +73,7 @@ impl<U, T: Clone + Add<T, Output=T>> Add for Length<U, T> {
 }
 
 // length += length
+#[cfg(not(feature="rustc_stable"))]
 impl<U, T: Clone + AddAssign<T>> AddAssign for Length<U, T> {
     fn add_assign(&mut self, other: Length<U, T>) {
         self.0 += other.get();
@@ -86,6 +89,7 @@ impl<U, T: Clone + Sub<T, Output=T>> Sub<Length<U, T>> for Length<U, T> {
 }
 
 // length -= length
+#[cfg(not(feature="rustc_stable"))]
 impl<U, T: Clone + SubAssign<T>> SubAssign for Length<U, T> {
     fn sub_assign(&mut self, other: Length<U, T>) {
         self.0 -= other.get();
@@ -160,10 +164,13 @@ impl<Unit, T: Clone + Ord> Ord for Length<Unit, T> {
     fn cmp(&self, other: &Length<Unit, T>) -> Ordering { self.get().cmp(&other.get()) }
 }
 
-impl<Unit, T: Zero> Zero for Length<Unit, T> {
+impl<Unit, T: Zero + Clone> Zero for Length<Unit, T> {
     fn zero() -> Length<Unit, T> {
         Length::new(Zero::zero())
     }
+    fn is_zero(&self) -> bool {
+        self.get().is_zero()
+    }
 }
 
 #[cfg(test)]
diff --git a/src/lib.rs b/src/lib.rs
index 392fa11..701074c 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,9 +7,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(asm, custom_derive, plugin, repr_simd, zero_one, test)]
-#![feature(augmented_assignments)]
-#![feature(op_assign_traits)]
+#![cfg_attr(not(feature="rustc_stable"), feature(asm, custom_derive, plugin, repr_simd, test))]
+#![cfg_attr(not(feature="rustc_stable"), feature(augmented_assignments))]
+#![cfg_attr(not(feature="rustc_stable"), feature(op_assign_traits))]
 
 #![cfg_attr(feature = "plugins", plugin(heapsize_plugin))]
 #![cfg_attr(feature = "plugins", plugin(serde_macros))]
@@ -25,6 +25,7 @@ extern crate rustc_serialize;
 extern crate serde;
 
 extern crate rand;
+#[cfg(not(feature="rustc_stable"))]
 extern crate test;
 extern crate num as num_lib;
 
@@ -32,7 +33,9 @@ pub use matrix::Matrix4;
 pub use matrix2d::Matrix2D;
 pub use point::{Point2D, Point3D, Point4D};
 pub use rect::Rect;
+#[cfg(not(feature="rustc_stable"))]
 pub use side_offsets::SideOffsets2D;
+#[cfg(not(feature="rustc_stable"))]
 pub use side_offsets::SideOffsets2DSimdI32;
 pub use size::Size2D;
 
@@ -40,9 +43,9 @@ pub mod approxeq;
 pub mod length;
 pub mod matrix;
 pub mod matrix2d;
-pub mod num;
 pub mod point;
 pub mod rect;
 pub mod scale_factor;
+#[cfg(not(feature="rustc_stable"))]
 pub mod side_offsets;
 pub mod size;
diff --git a/src/matrix2d.rs b/src/matrix2d.rs
index 0a62d94..0a2b097 100644
--- a/src/matrix2d.rs
+++ b/src/matrix2d.rs
@@ -7,7 +7,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use num::{One, Zero};
+use num_lib::{One, Zero};
 use point::Point2D;
 use rect::Rect;
 use size::Size2D;
diff --git a/src/num.rs b/src/num.rs
deleted file mode 100644
index 93c1bd3..0000000
--- a/src/num.rs
+++ /dev/null
@@ -1,29 +0,0 @@
-// Copyright 2014 The Servo Project Developers. See the COPYRIGHT
-// file at the top-level directory of this distribution.
-//
-// 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.
-//! A one-dimensional length, tagged with its units.
-
-use std::num;
-
-
-pub trait Zero {
-    fn zero() -> Self;
-}
-
-impl<T: num::Zero> Zero for T {
-    fn zero() -> T { num::Zero::zero() }
-}
-
-pub trait One {
-    fn one() -> Self;
-}
-
-impl<T: num::One> One for T {
-    fn one() -> T { num::One::one() }
-}
-
diff --git a/src/point.rs b/src/point.rs
index e14aebe..37a0230 100644
--- a/src/point.rs
+++ b/src/point.rs
@@ -9,8 +9,7 @@
 
 use length::Length;
 use size::Size2D;
-use num::Zero;
-
+use num_lib::Zero;
 use num_lib::NumCast;
 use num_lib::traits;
 use std::fmt::{self, Formatter};
diff --git a/src/rect.rs b/src/rect.rs
index 8b9a15f..e20b3d6 100644
--- a/src/rect.rs
+++ b/src/rect.rs
@@ -8,7 +8,7 @@
 // except according to those terms.
 
 use length::Length;
-use num::Zero;
+use num_lib::Zero;
 use point::Point2D;
 use size::Size2D;
 
diff --git a/src/scale_factor.rs b/src/scale_factor.rs
index c352b80..272aa6a 100644
--- a/src/scale_factor.rs
+++ b/src/scale_factor.rs
@@ -8,7 +8,7 @@
 // except according to those terms.
 //! A type-checked scaling factor between units.
 
-use num::One;
+use num_lib::One;
 
 use num_lib::NumCast;
 #[cfg(feature = "plugins")]
diff --git a/src/side_offsets.rs b/src/side_offsets.rs
index ddf5b67..e8d6412 100644
--- a/src/side_offsets.rs
+++ b/src/side_offsets.rs
@@ -10,7 +10,7 @@
 //! A group of side offsets, which correspond to top/left/bottom/right for borders, padding,
 //! and margins in CSS.
 
-use num::Zero;
+use num_lib::Zero;
 use std::ops::Add;
 
 /// A group of side offsets, which correspond to top/left/bottom/right for borders, padding,
diff --git a/src/size.rs b/src/size.rs
index 4c1ef61..60ae05d 100644
--- a/src/size.rs
+++ b/src/size.rs
@@ -8,11 +8,11 @@
 // except according to those terms.
 
 use length::Length;
-use num::Zero;
+use num_lib::Zero;
 
 use num_lib::NumCast;
 use std::fmt::{self, Formatter};
-use std::ops::{Mul, Div};
+use std::ops::{Add, Mul, Div};
 
 #[derive(Clone, Copy, RustcDecodable, RustcEncodable, PartialEq)]
 #[cfg_attr(feature = "plugins", derive(HeapSizeOf, Deserialize, Serialize))]
@@ -55,13 +55,26 @@ impl<T: Zero> Size2D<T> {
     }
 }
 
-impl<T: Zero> Zero for Size2D<T> {
+impl<T: Zero + Copy> Zero for Size2D<T> {
     fn zero() -> Size2D<T> {
         Size2D {
             width: Zero::zero(),
             height: Zero::zero(),
         }
     }
+    fn is_zero(&self) -> bool {
+        self.width.is_zero() && self.height.is_zero()
+    }
+}
+
+impl<T:Add<Output=T> + Copy> Add for Size2D<T> {
+    type Output = Size2D<T>;
+    fn add(self, rhs: Size2D<T>) -> Size2D<T> {
+        Size2D {
+            width: self.width + rhs.width,
+            height: self.width + rhs.width
+        }
+    }
 }
 
 impl<Scale: Copy, T0: Mul<Scale, Output=T1>, T1: Clone> Mul<Scale> for Size2D<T0> {