Use generics for the filter property

This introduces an additional shadow type for drop-shadow().

components/style/values/generics/effects.rs

@@ -0,0 +1,92 @@
+/* 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/. */
+
+//! Generic types for CSS values related to effects.
+
+use std::fmt;
+use style_traits::ToCss;
+#[cfg(feature = "gecko")]
+use values::specified::url::SpecifiedUrl;
+
+/// A generic value for the `filter` property.
+///
+/// Keyword `none` is represented by an empty slice.
+#[cfg_attr(feature = "servo", derive(Deserialize, HeapSizeOf, Serialize))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+pub struct FilterList<Filter>(pub Box<[Filter]>);
+
+/// A generic value for a single `filter`.
+#[cfg_attr(feature = "servo", derive(Deserialize, HeapSizeOf, Serialize))]
+#[derive(Clone, Debug, HasViewportPercentage, PartialEq, ToComputedValue, ToCss)]
+pub enum Filter<Angle, Factor, Length, DropShadow> {
+    /// `blur(<length>)`
+    #[css(function)]
+    Blur(Length),
+    /// `brightness(<factor>)`
+    #[css(function)]
+    Brightness(Factor),
+    /// `contrast(<factor>)`
+    #[css(function)]
+    Contrast(Factor),
+    /// `grayscale(<factor>)`
+    #[css(function)]
+    Grayscale(Factor),
+    /// `hue-rotate(<angle>)`
+    #[css(function)]
+    HueRotate(Angle),
+    /// `invert(<factor>)`
+    #[css(function)]
+    Invert(Factor),
+    /// `opacity(<factor>)`
+    #[css(function)]
+    Opacity(Factor),
+    /// `saturate(<factor>)`
+    #[css(function)]
+    Saturate(Factor),
+    /// `sepia(<factor>)`
+    #[css(function)]
+    Sepia(Factor),
+    /// `drop-shadow(...)`
+    #[css(function)]
+    DropShadow(DropShadow),
+    /// `<url>`
+    #[cfg(feature = "gecko")]
+    Url(SpecifiedUrl),
+}
+
+impl<F> FilterList<F> {
+    /// Returns `none`.
+    #[inline]
+    pub fn none() -> Self {
+        FilterList(vec![].into_boxed_slice())
+    }
+}
+
+impl<F> From<Vec<F>> for FilterList<F> {
+    #[inline]
+    fn from(vec: Vec<F>) -> Self {
+        FilterList(vec.into_boxed_slice())
+    }
+}
+
+impl<F> ToCss for FilterList<F>
+where
+    F: ToCss,
+{
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+    where
+        W: fmt::Write
+    {
+        if let Some((first, rest)) = self.0.split_first() {
+            first.to_css(dest)?;
+            for filter in rest {
+                dest.write_str(" ")?;
+                filter.to_css(dest)?;
+            }
+            Ok(())
+        } else {
+            dest.write_str("none")
+        }
+    }
+}