Use generics for background-size property

components/style/values/generics/background.rs

@@ -0,0 +1,52 @@
+/* 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 backgrounds.
+
+use std::fmt;
+use style_traits::ToCss;
+
+/// A generic value for the `background-size` property.
+#[derive(Clone, Copy, Debug, HasViewportPercentage, PartialEq, ToComputedValue)]
+#[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+pub enum BackgroundSize<LengthOrPercentageOrAuto> {
+    /// `<width> <height>`
+    Explicit {
+        /// Explicit width.
+        width: LengthOrPercentageOrAuto,
+        /// Explicit height.
+        height: LengthOrPercentageOrAuto
+    },
+    /// `cover`
+    Cover,
+    /// `contain`
+    Contain,
+}
+
+impl<L> From<L> for BackgroundSize<L>
+    where L: Clone,
+{
+    #[inline]
+    fn from(value: L) -> Self {
+        BackgroundSize::Explicit { width: value.clone(), height: value }
+    }
+}
+
+impl<L> ToCss for BackgroundSize<L>
+    where L: ToCss
+{
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result
+        where W: fmt::Write
+    {
+        match *self {
+            BackgroundSize::Explicit { ref width, ref height } => {
+                width.to_css(dest)?;
+                dest.write_str(" ")?;
+                height.to_css(dest)
+            },
+            BackgroundSize::Cover => dest.write_str("cover"),
+            BackgroundSize::Contain => dest.write_str("contain"),
+        }
+    }
+}

components/style/values/generics/mod.rs

@@ -15,6 +15,7 @@ use super::CustomIdent;
 
 pub use self::basic_shape::serialize_radius_values;
 
+pub mod background;
 pub mod basic_shape;
 pub mod border;
 pub mod grid;