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/length.rs

@@ -7,8 +7,8 @@
 use crate::parser::{Parse, ParserContext};
 #[cfg(feature = "gecko")]
 use crate::values::computed::ExtremumLength;
+use crate::Zero;
 use cssparser::Parser;
-use num_traits::Zero;
 use style_traits::ParseError;
 
 /// A `<length-percentage> | auto` value.
@@ -67,6 +67,19 @@ impl<LengthPercentage> LengthPercentageOrAuto<LengthPercentage> {
     }
 }
 
+impl<LengthPercentage: Zero> Zero for LengthPercentageOrAuto<LengthPercentage> {
+    fn zero() -> Self {
+        LengthPercentageOrAuto::LengthPercentage(Zero::zero())
+    }
+
+    fn is_zero(&self) -> bool {
+        match *self {
+            LengthPercentageOrAuto::LengthPercentage(ref l) => l.is_zero(),
+            LengthPercentageOrAuto::Auto => false,
+        }
+    }
+}
+
 impl<LengthPercentage: Parse> Parse for LengthPercentageOrAuto<LengthPercentage> {
     fn parse<'i, 't>(
         context: &ParserContext,
@@ -186,12 +199,15 @@ pub enum GenericLengthOrNumber<L, N> {
 
 pub use self::GenericLengthOrNumber as LengthOrNumber;
 
-impl<L, N> LengthOrNumber<L, N> {
-    /// Returns `0`.
-    pub fn zero() -> Self
-    where
-        N: Zero,
-    {
-        LengthOrNumber::Number(num_traits::Zero::zero())
+impl<L, N: Zero> Zero for LengthOrNumber<L, N> {
+    fn zero() -> Self {
+        LengthOrNumber::Number(Zero::zero())
+    }
+
+    fn is_zero(&self) -> bool {
+        match *self {
+            LengthOrNumber::Number(ref n) => n.is_zero(),
+            LengthOrNumber::Length(..) => false,
+        }
     }
 }