Auto merge of #17152 - servo:derive-all-the-things, r=emilio

Derive the most trivial ToCss implementations 🌋

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18 files changed, 113 insertions, 263 deletions

diff --git a/components/style/properties/helpers.mako.rs b/components/style/properties/helpers.mako.rs
index f815dc6cc49..c7031be3ebb 100644
--- a/components/style/properties/helpers.mako.rs
+++ b/components/style/properties/helpers.mako.rs
@@ -1074,8 +1074,6 @@
                           predefined_type=length_type,
                           logical=logical,
                           **kwargs)">
-        use std::fmt;
-        use style_traits::ToCss;
         % if not logical:
             use values::specified::AllowQuirks;
         % endif
@@ -1085,8 +1083,8 @@
             pub type T = ::values::computed::${length_type};
         }
 
-        #[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
         pub struct SpecifiedValue(pub ${length_type});
 
         % if length_type == "MozLength":
@@ -1131,12 +1129,6 @@
             ret.map(SpecifiedValue)
         }
 
-        impl ToCss for SpecifiedValue {
-            fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-                self.0.to_css(dest)
-            }
-        }
-
         impl ToComputedValue for SpecifiedValue {
             type ComputedValue = computed_value::T;
             #[inline]
diff --git a/components/style/properties/longhand/border.mako.rs b/components/style/properties/longhand/border.mako.rs
index f2eae0dfe42..693d1f32551 100644
--- a/components/style/properties/longhand/border.mako.rs
+++ b/components/style/properties/longhand/border.mako.rs
@@ -222,8 +222,8 @@ ${helpers.predefined_type("border-image-outset", "LengthOrNumberRect",
     pub mod computed_value {
         pub use super::RepeatKeyword;
 
-        #[derive(Debug, Clone, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Debug, Clone, PartialEq, ToCss)]
         pub struct T(pub RepeatKeyword, pub RepeatKeyword);
     }
 
@@ -238,14 +238,6 @@ ${helpers.predefined_type("border-image-outset", "LengthOrNumberRect",
                              "round" => Round,
                              "space" => Space);
 
-
-    impl ToCss for computed_value::T {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            try!(self.0.to_css(dest));
-            try!(dest.write_str(" "));
-            self.1.to_css(dest)
-        }
-    }
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
             try!(self.0.to_css(dest));
diff --git a/components/style/properties/longhand/color.mako.rs b/components/style/properties/longhand/color.mako.rs
index 238b8689e26..0728516ab41 100644
--- a/components/style/properties/longhand/color.mako.rs
+++ b/components/style/properties/longhand/color.mako.rs
@@ -13,8 +13,6 @@
                    ignored_when_colors_disabled="True"
                    spec="https://drafts.csswg.org/css-color/#color">
     use cssparser::RGBA;
-    use std::fmt;
-    use style_traits::ToCss;
     use values::specified::{AllowQuirks, Color, CSSColor};
 
     impl ToComputedValue for SpecifiedValue {
@@ -31,17 +29,11 @@
         }
     }
 
-    #[derive(Clone, PartialEq, Debug)]
     #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+    #[derive(Clone, Debug, PartialEq, ToCss)]
     pub struct SpecifiedValue(pub CSSColor);
     no_viewport_percentage!(SpecifiedValue);
 
-    impl ToCss for SpecifiedValue {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            self.0.to_css(dest)
-        }
-    }
-
     pub mod computed_value {
         use cssparser;
         pub type T = cssparser::RGBA;
@@ -91,6 +83,9 @@
         %>
         use gecko_bindings::bindings::Gecko_GetLookAndFeelSystemColor;
         use gecko_bindings::structs::root::mozilla::LookAndFeel_ColorID;
+        use std::fmt;
+        use style_traits::ToCss;
+
         pub type SystemColor = LookAndFeel_ColorID;
 
         impl ToCss for SystemColor {
diff --git a/components/style/properties/longhand/font.mako.rs b/components/style/properties/longhand/font.mako.rs
index 37410b1be82..2c491a1630a 100644
--- a/components/style/properties/longhand/font.mako.rs
+++ b/components/style/properties/longhand/font.mako.rs
@@ -2149,18 +2149,15 @@ ${helpers.single_keyword("-moz-math-variant",
                    internal="True" disable_when_testing="True">
     use app_units::Au;
     use gecko_bindings::structs::NS_MATHML_DEFAULT_SCRIPT_MIN_SIZE_PT;
-    use std::fmt;
-    use style_traits::ToCss;
     use values::specified::length::{AU_PER_PT, FontBaseSize, NoCalcLength};
 
-    #[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
+    #[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
     pub struct SpecifiedValue(pub NoCalcLength);
 
     pub mod computed_value {
         pub type T = super::Au;
     }
 
-
     impl ToComputedValue for SpecifiedValue {
         type ComputedValue = computed_value::T;
 
@@ -2185,12 +2182,6 @@ ${helpers.single_keyword("-moz-math-variant",
         }
     }
 
-    impl ToCss for SpecifiedValue {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            self.0.to_css(dest)
-        }
-    }
-
     #[inline]
     pub fn get_initial_value() -> computed_value::T {
         Au((NS_MATHML_DEFAULT_SCRIPT_MIN_SIZE_PT as f32 * AU_PER_PT) as i32)
diff --git a/components/style/properties/longhand/inherited_table.mako.rs b/components/style/properties/longhand/inherited_table.mako.rs
index 424d74a5b4c..1cf2c14fbe0 100644
--- a/components/style/properties/longhand/inherited_table.mako.rs
+++ b/components/style/properties/longhand/inherited_table.mako.rs
@@ -31,8 +31,8 @@ ${helpers.single_keyword("caption-side", "top bottom",
         use app_units::Au;
         use properties::animated_properties::Animatable;
 
-        #[derive(Clone, Copy, Debug, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Clone, Copy, Debug, PartialEq, ToCss)]
         pub struct T {
             pub horizontal: Au,
             pub vertical: Au,
@@ -92,14 +92,6 @@ ${helpers.single_keyword("caption-side", "top bottom",
         }
     }
 
-    impl ToCss for computed_value::T {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            try!(self.horizontal.to_css(dest));
-            try!(dest.write_str(" "));
-            self.vertical.to_css(dest)
-        }
-    }
-
     impl ToComputedValue for SpecifiedValue {
         type ComputedValue = computed_value::T;
 
diff --git a/components/style/properties/longhand/inherited_text.mako.rs b/components/style/properties/longhand/inherited_text.mako.rs
index 8b1dafb5cd4..690d50d7908 100644
--- a/components/style/properties/longhand/inherited_text.mako.rs
+++ b/components/style/properties/longhand/inherited_text.mako.rs
@@ -443,8 +443,8 @@ ${helpers.predefined_type("word-spacing",
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
         pub struct T(pub SmallVec<[TextShadow; 1]>);
 
-        #[derive(Clone, PartialEq, Debug)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Clone, PartialEq, Debug, ToCss)]
         pub struct TextShadow {
             pub offset_x: Au,
             pub offset_y: Au,
@@ -468,18 +468,6 @@ ${helpers.predefined_type("word-spacing",
         }
     }
 
-    impl ToCss for computed_value::TextShadow {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            self.offset_x.to_css(dest)?;
-            dest.write_str(" ")?;
-            self.offset_y.to_css(dest)?;
-            dest.write_str(" ")?;
-            self.blur_radius.to_css(dest)?;
-            dest.write_str(" ")?;
-            self.color.to_css(dest)
-        }
-    }
-
     impl ToCss for SpecifiedValue {
         fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
             let mut iter = self.0.iter();
@@ -767,7 +755,6 @@ ${helpers.predefined_type("word-spacing",
 
 <%helpers:longhand name="text-emphasis-position" animation_value_type="none" products="gecko"
                    spec="https://drafts.csswg.org/css-text-decor/#propdef-text-emphasis-position">
-    use std::fmt;
     use values::computed::ComputedValueAsSpecified;
     use style_traits::ToCss;
 
@@ -778,8 +765,8 @@ ${helpers.predefined_type("word-spacing",
                              "right" => Right,
                              "left" => Left);
 
-    #[derive(Debug, Clone, PartialEq)]
     #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+    #[derive(Debug, Clone, PartialEq, ToCss)]
     pub struct SpecifiedValue(pub HorizontalWritingModeValue, pub VerticalWritingModeValue);
 
     pub mod computed_value {
@@ -804,15 +791,6 @@ ${helpers.predefined_type("word-spacing",
         }
     }
 
-    impl ToCss for SpecifiedValue {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            let SpecifiedValue(horizontal, vertical) = *self;
-            try!(horizontal.to_css(dest));
-            try!(dest.write_char(' '));
-            vertical.to_css(dest)
-        }
-    }
-
     % if product == "gecko":
         impl SpecifiedValue {
             pub fn from_gecko_keyword(kw: u32) -> Self {
diff --git a/components/style/properties/longhand/list.mako.rs b/components/style/properties/longhand/list.mako.rs
index 01b19d7d4fd..34f51b97b9d 100644
--- a/components/style/properties/longhand/list.mako.rs
+++ b/components/style/properties/longhand/list.mako.rs
@@ -114,17 +114,15 @@ ${helpers.single_keyword("list-style-position", "outside inside", animation_valu
 <%helpers:longhand name="list-style-image" animation_value_type="none"
                    boxed="${product == 'gecko'}"
                    spec="https://drafts.csswg.org/css-lists/#propdef-list-style-image">
-    use std::fmt;
     use values::computed::ComputedValueAsSpecified;
     use values::specified::UrlOrNone;
     pub use self::computed_value::T as SpecifiedValue;
-    use style_traits::ToCss;
 
     pub mod computed_value {
         use values::specified::UrlOrNone;
 
-        #[derive(Debug, Clone, PartialEq)]
         #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+        #[derive(Debug, Clone, PartialEq, ToCss)]
         pub struct T(pub UrlOrNone);
     }
 
@@ -132,12 +130,6 @@ ${helpers.single_keyword("list-style-position", "outside inside", animation_valu
     impl ComputedValueAsSpecified for SpecifiedValue {}
     no_viewport_percentage!(SpecifiedValue);
 
-    impl ToCss for SpecifiedValue {
-        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-            self.0.to_css(dest)
-        }
-    }
-
     #[inline]
     pub fn get_initial_value() -> computed_value::T {
         computed_value::T(Either::Second(None_))
diff --git a/components/style/values/computed/length.rs b/components/style/values/computed/length.rs
index e5b35428bef..3d59a51ff42 100644
--- a/components/style/values/computed/length.rs
+++ b/components/style/values/computed/length.rs
@@ -623,9 +623,9 @@ pub type LengthOrNormal = Either<Length, Normal>;
 
 /// A value suitable for a `min-width`, `min-height`, `width` or `height` property.
 /// See specified/values/length.rs for more details.
-#[derive(Debug, Copy, Clone, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum MozLength {
     LengthOrPercentageOrAuto(LengthOrPercentageOrAuto),
     ExtremumLength(ExtremumLength),
@@ -665,22 +665,11 @@ impl ToComputedValue for specified::MozLength {
     }
 }
 
-impl ToCss for MozLength {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            MozLength::LengthOrPercentageOrAuto(lopoa) =>
-                lopoa.to_css(dest),
-            MozLength::ExtremumLength(ext) =>
-                ext.to_css(dest),
-        }
-    }
-}
-
 /// A value suitable for a `max-width` or `max-height` property.
 /// See specified/values/length.rs for more details.
-#[derive(Debug, Copy, Clone, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum MaxLength {
     LengthOrPercentageOrNone(LengthOrPercentageOrNone),
     ExtremumLength(ExtremumLength),
@@ -718,14 +707,3 @@ impl ToComputedValue for specified::MaxLength {
         }
     }
 }
-
-impl ToCss for MaxLength {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            MaxLength::LengthOrPercentageOrNone(lopon) =>
-                lopon.to_css(dest),
-            MaxLength::ExtremumLength(ext) =>
-                ext.to_css(dest),
-        }
-    }
-}
diff --git a/components/style/values/computed/mod.rs b/components/style/values/computed/mod.rs
index 039c3763567..1fbce99389e 100644
--- a/components/style/values/computed/mod.rs
+++ b/components/style/values/computed/mod.rs
@@ -481,9 +481,9 @@ pub struct Shadow {
 /// A `<number>` value.
 pub type Number = CSSFloat;
 
-#[derive(Debug, Copy, Clone, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum NumberOrPercentage {
     Percentage(Percentage),
     Number(Number),
@@ -512,15 +512,6 @@ impl ToComputedValue for specified::NumberOrPercentage {
     }
 }
 
-impl ToCss for NumberOrPercentage {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            NumberOrPercentage::Percentage(percentage) => percentage.to_css(dest),
-            NumberOrPercentage::Number(number) => number.to_css(dest),
-        }
-    }
-}
-
 /// A type used for opacity.
 pub type Opacity = CSSFloat;
 
diff --git a/components/style/values/generics/basic_shape.rs b/components/style/values/generics/basic_shape.rs
index a507d15a4ec..b2d402dd38f 100644
--- a/components/style/values/generics/basic_shape.rs
+++ b/components/style/values/generics/basic_shape.rs
@@ -53,7 +53,7 @@ pub enum ShapeSource<BasicShape, ReferenceBox> {
 
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Debug, PartialEq, ToComputedValue)]
+#[derive(Clone, Debug, PartialEq, ToComputedValue, ToCss)]
 pub enum BasicShape<H, V, LengthOrPercentage> {
     Inset(InsetRect<LengthOrPercentage>),
     Circle(Circle<H, V, LengthOrPercentage>),
@@ -153,23 +153,6 @@ impl ToCss for GeometryBox {
     }
 }
 
-impl<H, V, L> ToCss for BasicShape<H, V, L>
-    where H: ToCss,
-          V: ToCss,
-          L: PartialEq + ToCss,
-          Circle<H, V, L>: ToCss,
-          Ellipse<H, V, L>: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            BasicShape::Inset(ref rect) => rect.to_css(dest),
-            BasicShape::Circle(ref circle) => circle.to_css(dest),
-            BasicShape::Ellipse(ref ellipse) => ellipse.to_css(dest),
-            BasicShape::Polygon(ref polygon) => polygon.to_css(dest),
-        }
-    }
-}
-
 impl<L> ToCss for InsetRect<L>
     where L: ToCss + PartialEq
 {
diff --git a/components/style/values/generics/image.rs b/components/style/values/generics/image.rs
index 9aa49428dce..5965db357dc 100644
--- a/components/style/values/generics/image.rs
+++ b/components/style/values/generics/image.rs
@@ -108,8 +108,8 @@ impl ComputedValueAsSpecified for ShapeExtent {}
 
 /// A gradient item.
 /// https://drafts.csswg.org/css-images-4/#color-stop-syntax
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 pub enum GradientItem<Color, LengthOrPercentage> {
     /// A color stop.
     ColorStop(ColorStop<Color, LengthOrPercentage>),
@@ -288,17 +288,6 @@ impl<L, LoP> ToCss for EndingShape<L, LoP>
     }
 }
 
-impl<C, L> ToCss for GradientItem<C, L>
-    where C: ToCss, L: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            GradientItem::ColorStop(ref stop) => stop.to_css(dest),
-            GradientItem::InterpolationHint(ref hint) => hint.to_css(dest),
-        }
-    }
-}
-
 impl<C, L> fmt::Debug for ColorStop<C, L>
     where C: fmt::Debug, L: fmt::Debug,
 {
diff --git a/components/style/values/generics/transform.rs b/components/style/values/generics/transform.rs
index 7cbc9ce4521..9fe5a8cab93 100644
--- a/components/style/values/generics/transform.rs
+++ b/components/style/values/generics/transform.rs
@@ -4,12 +4,9 @@
 
 //! Generic types for CSS values that are related to transformations.
 
-use std::fmt;
-use style_traits::ToCss;
-
 /// A generic transform origin.
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
-#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
 pub struct TransformOrigin<H, V, Depth> {
     /// The horizontal origin.
     pub horizontal: H,
@@ -29,17 +26,3 @@ impl<H, V, D> TransformOrigin<H, V, D> {
         }
     }
 }
-
-impl<H, V, D> ToCss for TransformOrigin<H, V, D>
-    where H: ToCss, V: ToCss, D: ToCss,
-{
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
-        where W: fmt::Write,
-    {
-        self.horizontal.to_css(dest)?;
-        dest.write_str(" ")?;
-        self.vertical.to_css(dest)?;
-        dest.write_str(" ")?;
-        self.depth.to_css(dest)
-    }
-}
diff --git a/components/style/values/mod.rs b/components/style/values/mod.rs
index bbb6603bb90..506584c1e76 100644
--- a/components/style/values/mod.rs
+++ b/components/style/values/mod.rs
@@ -34,9 +34,9 @@ define_keyword_type!(None_, "none");
 define_keyword_type!(Auto, "auto");
 define_keyword_type!(Normal, "normal");
 
-#[derive(Clone, Copy, HasViewportPercentage, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 /// A struct representing one of two kinds of values.
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, HasViewportPercentage, PartialEq, ToCss)]
 pub enum Either<A, B> {
     /// The first value.
     First(A),
@@ -53,15 +53,6 @@ impl<A: Debug, B: Debug> Debug for Either<A, B> {
     }
 }
 
-impl<A: ToCss, B: ToCss> ToCss for Either<A, B> {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            Either::First(ref v) => v.to_css(dest),
-            Either::Second(ref v) => v.to_css(dest),
-        }
-    }
-}
-
 impl<A: Parse, B: Parse> Parse for Either<A, B> {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Either<A, B>, ()> {
         if let Ok(v) = input.try(|i| A::parse(context, i)) {
diff --git a/components/style/values/specified/align.rs b/components/style/values/specified/align.rs
index c0e34a2f2a2..6fb637b6942 100644
--- a/components/style/values/specified/align.rs
+++ b/components/style/values/specified/align.rs
@@ -209,7 +209,7 @@ impl Parse for AlignJustifyContent {
 /// Value of the `align-self` or `justify-self` property.
 ///
 /// https://drafts.csswg.org/css-align/#self-alignment
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, ToCss)]
 pub struct AlignJustifySelf(pub AlignFlags);
 
 impl AlignJustifySelf {
@@ -228,12 +228,6 @@ impl AlignJustifySelf {
 
 no_viewport_percentage!(AlignJustifySelf);
 
-impl ToCss for AlignJustifySelf {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        self.0.to_css(dest)
-    }
-}
-
 impl Parse for AlignJustifySelf {
     // auto | normal | stretch | <baseline-position> |
     // [ <overflow-position>? && <self-position> ]
@@ -253,7 +247,7 @@ impl Parse for AlignJustifySelf {
 /// Value of the `align-items` property
 ///
 /// https://drafts.csswg.org/css-align/#self-alignment
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, ToCss)]
 pub struct AlignItems(pub AlignFlags);
 
 impl AlignItems {
@@ -272,12 +266,6 @@ impl AlignItems {
 
 no_viewport_percentage!(AlignItems);
 
-impl ToCss for AlignItems {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        self.0.to_css(dest)
-    }
-}
-
 impl Parse for AlignItems {
     // normal | stretch | <baseline-position> |
     // [ <overflow-position>? && <self-position> ]
@@ -297,7 +285,7 @@ impl Parse for AlignItems {
 /// Value of the `justify-items` property
 ///
 /// https://drafts.csswg.org/css-align/#justify-items-property
-#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq, ToCss)]
 pub struct JustifyItems(pub AlignFlags);
 
 impl JustifyItems {
@@ -316,12 +304,6 @@ impl JustifyItems {
 
 no_viewport_percentage!(JustifyItems);
 
-impl ToCss for JustifyItems {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        self.0.to_css(dest)
-    }
-}
-
 impl Parse for JustifyItems {
     // auto | normal | stretch | <baseline-position> |
     // [ <overflow-position>? && <self-position> ]
diff --git a/components/style/values/specified/length.rs b/components/style/values/specified/length.rs
index 6273fec31fd..61335ee0e55 100644
--- a/components/style/values/specified/length.rs
+++ b/components/style/values/specified/length.rs
@@ -531,8 +531,8 @@ impl NoCalcLength {
 /// This is commonly used for the `<length>` values.
 ///
 /// https://drafts.csswg.org/css-values/#lengths
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum Length {
     /// The internal length type that cannot parse `calc`
     NoCalc(NoCalcLength),
@@ -549,15 +549,6 @@ impl From<NoCalcLength> for Length {
     }
 }
 
-impl ToCss for Length {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            Length::NoCalc(ref inner) => inner.to_css(dest),
-            Length::Calc(ref calc) => calc.to_css(dest),
-        }
-    }
-}
-
 impl Mul<CSSFloat> for Length {
     type Output = Length;
 
@@ -758,9 +749,9 @@ impl Parse for Percentage {
 impl ComputedValueAsSpecified for Percentage {}
 
 /// A length or a percentage value.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum LengthOrPercentage {
     Length(NoCalcLength),
     Percentage(Percentage),
@@ -790,16 +781,6 @@ impl From<Percentage> for LengthOrPercentage {
     }
 }
 
-impl ToCss for LengthOrPercentage {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            LengthOrPercentage::Length(ref length) => length.to_css(dest),
-            LengthOrPercentage::Percentage(percentage) => percentage.to_css(dest),
-            LengthOrPercentage::Calc(ref calc) => calc.to_css(dest),
-        }
-    }
-}
-
 impl LengthOrPercentage {
     #[inline]
     /// Returns a `zero` length.
@@ -1213,25 +1194,14 @@ impl LengthOrNumber {
 /// A value suitable for a `min-width` or `min-height` property.
 /// Unlike `max-width` or `max-height` properties, a MozLength can be
 /// `auto`, and cannot be `none`.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum MozLength {
     LengthOrPercentageOrAuto(LengthOrPercentageOrAuto),
     ExtremumLength(ExtremumLength),
 }
 
-impl ToCss for MozLength {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            MozLength::LengthOrPercentageOrAuto(ref lopoa) =>
-                lopoa.to_css(dest),
-            MozLength::ExtremumLength(ref ext) =>
-                ext.to_css(dest),
-        }
-    }
-}
-
 impl Parse for MozLength {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         MozLength::parse_quirky(context, input, AllowQuirks::No)
@@ -1250,25 +1220,14 @@ impl MozLength {
 }
 
 /// A value suitable for a `max-width` or `max-height` property.
-#[derive(Clone, Debug, HasViewportPercentage, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToCss)]
 pub enum MaxLength {
     LengthOrPercentageOrNone(LengthOrPercentageOrNone),
     ExtremumLength(ExtremumLength),
 }
 
-impl ToCss for MaxLength {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            MaxLength::LengthOrPercentageOrNone(ref lopon) =>
-                lopon.to_css(dest),
-            MaxLength::ExtremumLength(ref ext) =>
-                ext.to_css(dest),
-        }
-    }
-}
-
 impl Parse for MaxLength {
     fn parse(context: &ParserContext, input: &mut Parser) -> Result<Self, ()> {
         MaxLength::parse_quirky(context, input, AllowQuirks::No)
diff --git a/components/style/values/specified/mod.rs b/components/style/values/specified/mod.rs
index 7bd45a3e753..2b9f3eddefa 100644
--- a/components/style/values/specified/mod.rs
+++ b/components/style/values/specified/mod.rs
@@ -652,9 +652,9 @@ impl ToCss for Number {
 
 /// <number-percentage>
 /// Accepts only non-negative numbers.
-#[derive(Clone, Copy, Debug, PartialEq)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, ToCss)]
 pub enum NumberOrPercentage {
     Percentage(Percentage),
     Number(Number),
@@ -686,18 +686,9 @@ impl Parse for NumberOrPercentage {
     }
 }
 
-impl ToCss for NumberOrPercentage {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        match *self {
-            NumberOrPercentage::Percentage(percentage) => percentage.to_css(dest),
-            NumberOrPercentage::Number(number) => number.to_css(dest),
-        }
-    }
-}
-
-#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
-#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+#[derive(Clone, Copy, Debug, PartialEq, PartialOrd, ToCss)]
 pub struct Opacity(Number);
 
 no_viewport_percentage!(Opacity);
@@ -722,12 +713,6 @@ impl ToComputedValue for Opacity {
     }
 }
 
-impl ToCss for Opacity {
-    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
-        self.0.to_css(dest)
-    }
-}
-
 #[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 #[allow(missing_docs)]
diff --git a/components/style_derive/lib.rs b/components/style_derive/lib.rs
index 7f4743dccd7..0f39a7deca1 100644
--- a/components/style_derive/lib.rs
+++ b/components/style_derive/lib.rs
@@ -11,6 +11,7 @@ use proc_macro::TokenStream;
 
 mod has_viewport_percentage;
 mod to_computed_value;
+mod to_css;
 
 #[proc_macro_derive(HasViewportPercentage)]
 pub fn derive_has_viewport_percentage(stream: TokenStream) -> TokenStream {
@@ -23,3 +24,9 @@ pub fn derive_to_computed_value(stream: TokenStream) -> TokenStream {
     let input = syn::parse_derive_input(&stream.to_string()).unwrap();
     to_computed_value::derive(input).to_string().parse().unwrap()
 }
+
+#[proc_macro_derive(ToCss)]
+pub fn derive_to_css(stream: TokenStream) -> TokenStream {
+    let input = syn::parse_derive_input(&stream.to_string()).unwrap();
+    to_css::derive(input).to_string().parse().unwrap()
+}
diff --git a/components/style_derive/to_css.rs b/components/style_derive/to_css.rs
new file mode 100644
index 00000000000..774add083fa
--- /dev/null
+++ b/components/style_derive/to_css.rs
@@ -0,0 +1,70 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use quote;
+use syn;
+use synstructure;
+
+pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut where_clause = where_clause.clone();
+    for param in &input.generics.ty_params {
+        where_clause.predicates.push(where_predicate(syn::Ty::Path(None, param.ident.clone().into())))
+    }
+
+    let style = synstructure::BindStyle::Ref.into();
+    let match_body = synstructure::each_variant(&input, &style, |bindings, _| {
+        if bindings.is_empty() {
+            panic!("unit variants are not yet supported");
+        }
+        let (first, rest) = bindings.split_first().expect("unit variants are not yet supported");
+        where_clause.predicates.push(where_predicate(first.field.ty.clone()));
+        let mut expr = quote! {
+            ::style_traits::ToCss::to_css(#first, dest)
+        };
+        for binding in rest {
+            where_clause.predicates.push(where_predicate(binding.field.ty.clone()));
+            expr = quote! {
+                #expr?;
+                dest.write_str(" ")?;
+                ::style_traits::ToCss::to_css(#binding, dest)
+            };
+        }
+        Some(expr)
+    });
+
+    quote! {
+        impl #impl_generics ::style_traits::ToCss for #name #ty_generics #where_clause {
+            #[allow(unused_variables, unused_imports)]
+            #[inline]
+            fn to_css<W>(&self, dest: &mut W) -> ::std::fmt::Result
+            where
+                W: ::std::fmt::Write
+            {
+                match *self {
+                    #match_body
+                }
+            }
+        }
+    }
+}
+
+/// `#ty: ::style_traits::ToCss`
+fn where_predicate(ty: syn::Ty) -> syn::WherePredicate {
+    syn::WherePredicate::BoundPredicate(syn::WhereBoundPredicate {
+        bound_lifetimes: vec![],
+        bounded_ty: ty,
+        bounds: vec![syn::TyParamBound::Trait(
+            syn::PolyTraitRef {
+                bound_lifetimes: vec![],
+                trait_ref: syn::Path {
+                    global: true,
+                    segments: vec!["style_traits".into(), "ToCss".into()],
+                },
+            },
+            syn::TraitBoundModifier::None
+        )],
+    })
+}