style: Use derive for -x-span.

components/style/macros.rs

@@ -4,6 +4,24 @@
 
 //! Various macro helpers.
 
+macro_rules! trivial_to_computed_value {
+    ($name: ident) => {
+        impl $crate::values::computed::ToComputedValue for $name {
+            type ComputedValue = $name;
+
+            fn to_computed_value(&self, _: &$crate::values::computed::Context) -> Self {
+                self.clone()
+            }
+
+            fn from_computed_value(other: &Self) -> Self {
+                other.clone()
+            }
+        }
+    }
+}
+
+trivial_to_computed_value!(i32);
+
 /// A macro to parse an identifier, or return an `UnexpectedIndent` error
 /// otherwise.
 ///
@@ -76,17 +94,7 @@ macro_rules! add_impls_for_keyword_enum {
             }
         }
 
-        impl $crate::values::computed::ToComputedValue for $name {
-            type ComputedValue = $name;
-
-            fn to_computed_value(&self, _: &$crate::values::computed::Context) -> Self {
-                self.clone()
-            }
-
-            fn from_computed_value(other: &Self) -> Self {
-                other.clone()
-            }
-        }
+        trivial_to_computed_value!($name);
     };
 }
 

components/style/properties/longhand/table.mako.rs

@@ -14,15 +14,12 @@ ${helpers.single_keyword("table-layout", "auto fixed",
                    spec="Internal-only (for `<col span>` pres attr)"
                    animation_value_type="none"
                    internal="True">
-    use values::computed::ComputedValueAsSpecified;
-
-    impl ComputedValueAsSpecified for SpecifiedValue {}
     pub type SpecifiedValue = computed_value::T;
     pub mod computed_value {
         use std::fmt;
         use style_traits::ToCss;
 
-        #[derive(Clone, Copy, Debug, PartialEq)]
+        #[derive(Clone, Copy, Debug, PartialEq, ToComputedValue)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub i32);
 
@@ -39,8 +36,10 @@ ${helpers.single_keyword("table-layout", "auto fixed",
     }
 
     // never parse it, only set via presentation attribute
-    fn parse<'i, 't>(_: &ParserContext, _: &mut Parser<'i, 't>)
-                     -> Result<SpecifiedValue, ParseError<'i>> {
+    fn parse<'i, 't>(
+        _: &ParserContext,
+        _: &mut Parser<'i, 't>,
+    ) -> Result<SpecifiedValue, ParseError<'i>> {
         Err(StyleParseError::UnspecifiedError.into())
     }
 </%helpers:longhand>