style: Add a Zero trait that doesn't require Add, and use it in place of num_traits and IsZeroLength.

Use it to be consistent in InsetRect serialization and storage between Servo and
Gecko.

Differential Revision: https://phabricator.services.mozilla.com/D21493

components/style/values/generics/border.rs

@@ -6,6 +6,7 @@
 
 use crate::values::generics::rect::Rect;
 use crate::values::generics::size::Size2D;
+use crate::Zero;
 use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 
@@ -65,6 +66,16 @@ impl<L> BorderCornerRadius<L> {
     }
 }
 
+impl<L: Zero> Zero for BorderCornerRadius<L> {
+    fn zero() -> Self {
+        BorderCornerRadius(Size2D::zero())
+    }
+
+    fn is_zero(&self) -> bool {
+        self.0.is_zero()
+    }
+}
+
 /// A generic value for the `border-spacing` property.
 #[derive(
     Animate,
@@ -135,12 +146,7 @@ impl<L> BorderRadius<L> {
             bottom_left: bl,
         }
     }
-}
 
-impl<L> BorderRadius<L>
-where
-    L: PartialEq + ToCss,
-{
     /// Serialises two given rects following the syntax of the `border-radius``
     /// property.
     pub fn serialize_rects<W>(
@@ -149,6 +155,7 @@ where
         dest: &mut CssWriter<W>,
     ) -> fmt::Result
     where
+        L: PartialEq + ToCss,
         W: Write,
     {
         widths.to_css(dest)?;
@@ -160,6 +167,24 @@ where
     }
 }
 
+impl<L: Zero> Zero for BorderRadius<L> {
+    fn zero() -> Self {
+        Self::new(
+            BorderCornerRadius::<L>::zero(),
+            BorderCornerRadius::<L>::zero(),
+            BorderCornerRadius::<L>::zero(),
+            BorderCornerRadius::<L>::zero(),
+        )
+    }
+
+    fn is_zero(&self) -> bool {
+        self.top_left.is_zero() &&
+            self.top_right.is_zero() &&
+            self.bottom_right.is_zero() &&
+            self.bottom_left.is_zero()
+    }
+}
+
 impl<L> ToCss for BorderRadius<L>
 where
     L: PartialEq + ToCss,