Properly implement ToAnimatedZero for BorderSpacing

components/style/values/animated/mod.rs

@@ -380,3 +380,16 @@ where
         }
     }
 }
+
+impl<T> ToAnimatedZero for Size2D<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        Ok(Size2D::new(
+            self.width.to_animated_zero()?,
+            self.height.to_animated_zero()?,
+        ))
+    }
+}

components/style/values/computed/border.rs

@@ -69,14 +69,6 @@ impl BorderCornerRadius {
     }
 }
 
-impl ToAnimatedZero for BorderSpacing {
-    #[inline]
-    fn to_animated_zero(&self) -> Result<Self, ()> {
-        // FIXME(emilio): Why?
-        Err(())
-    }
-}
-
 impl ToAnimatedZero for BorderCornerRadius {
     #[inline]
     fn to_animated_zero(&self) -> Result<Self, ()> {

components/style/values/generics/border.rs

@@ -43,7 +43,7 @@ impl<L> BorderCornerRadius<L> {
 
 /// A generic value for the `border-spacing` property.
 #[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug, MallocSizeOf)]
-#[derive(PartialEq, ToAnimatedValue, ToComputedValue, ToCss)]
+#[derive(PartialEq, ToAnimatedValue, ToAnimatedZero, ToComputedValue, ToCss)]
 pub struct BorderSpacing<L>(pub Size<L>);
 
 impl<L> BorderSpacing<L> {

components/style/values/generics/size.rs

@@ -14,7 +14,7 @@ use values::animated::ToAnimatedValue;
 /// A generic size, for `border-*-radius` longhand properties, or
 /// `border-spacing`.
 #[derive(Animate, Clone, ComputeSquaredDistance, Copy, Debug)]
-#[derive(MallocSizeOf, PartialEq, ToComputedValue)]
+#[derive(MallocSizeOf, PartialEq, ToAnimatedZero, ToComputedValue)]
 pub struct Size<L>(pub Size2D<L>);
 
 impl<L> Size<L> {