style: Use rust types for background-size.

Hopefully straight-forward.

Differential Revision: https://phabricator.services.mozilla.com/D19625

components/style/values/computed/background.rs

@@ -6,19 +6,8 @@
 
 use crate::values::computed::length::NonNegativeLengthPercentage;
 use crate::values::generics::background::BackgroundSize as GenericBackgroundSize;
-use crate::values::generics::length::LengthPercentageOrAuto;
 
 pub use crate::values::specified::background::BackgroundRepeat;
 
 /// A computed value for the `background-size` property.
 pub type BackgroundSize = GenericBackgroundSize<NonNegativeLengthPercentage>;
-
-impl BackgroundSize {
-    /// Returns `auto auto`.
-    pub fn auto() -> Self {
-        GenericBackgroundSize::Explicit {
-            width: LengthPercentageOrAuto::auto(),
-            height: LengthPercentageOrAuto::auto(),
-        }
-    }
-}

components/style/values/generics/background.rs

@@ -4,7 +4,7 @@
 
 //! Generic types for CSS values related to backgrounds.
 
-use crate::values::generics::length::LengthPercentageOrAuto;
+use crate::values::generics::length::{LengthPercentageOrAuto, GenericLengthPercentageOrAuto};
 use std::fmt::{self, Write};
 use style_traits::{CssWriter, ToCss};
 
@@ -22,13 +22,14 @@ use style_traits::{CssWriter, ToCss};
     ToAnimatedZero,
     ToComputedValue,
 )]
-pub enum BackgroundSize<LengthPercentage> {
+#[repr(C, u8)]
+pub enum GenericBackgroundSize<LengthPercent> {
     /// `<width> <height>`
-    Explicit {
+    ExplicitSize {
         /// Explicit width.
-        width: LengthPercentageOrAuto<LengthPercentage>,
+        width: GenericLengthPercentageOrAuto<LengthPercent>,
         /// Explicit height.
-        height: LengthPercentageOrAuto<LengthPercentage>,
+        height: GenericLengthPercentageOrAuto<LengthPercent>,
     },
     /// `cover`
     #[animation(error)]
@@ -38,6 +39,8 @@ pub enum BackgroundSize<LengthPercentage> {
     Contain,
 }
 
+pub use self::GenericBackgroundSize as BackgroundSize;
+
 impl<LengthPercentage> ToCss for BackgroundSize<LengthPercentage>
 where
     LengthPercentage: ToCss,
@@ -47,7 +50,7 @@ where
         W: Write,
     {
         match self {
-            BackgroundSize::Explicit { width, height } => {
+            BackgroundSize::ExplicitSize { width, height } => {
                 width.to_css(dest)?;
                 // NOTE(emilio): We should probably simplify all these in case
                 // `width == `height`, but all other browsers agree on only
@@ -63,3 +66,13 @@ where
         }
     }
 }
+
+impl<LengthPercentage> BackgroundSize<LengthPercentage> {
+    /// Returns `auto auto`.
+    pub fn auto() -> Self {
+        GenericBackgroundSize::ExplicitSize {
+            width: LengthPercentageOrAuto::Auto,
+            height: LengthPercentageOrAuto::Auto,
+        }
+    }
+}

components/style/values/specified/background.rs

@@ -26,7 +26,7 @@ impl Parse for BackgroundSize {
             let height = input
                 .try(|i| NonNegativeLengthPercentageOrAuto::parse(context, i))
                 .unwrap_or(NonNegativeLengthPercentageOrAuto::auto());
-            return Ok(GenericBackgroundSize::Explicit { width, height });
+            return Ok(GenericBackgroundSize::ExplicitSize { width, height });
         }
         Ok(try_match_ident_ignore_ascii_case! { input,
             "cover" => GenericBackgroundSize::Cover,
@@ -35,16 +35,6 @@ impl Parse for BackgroundSize {
     }
 }
 
-impl BackgroundSize {
-    /// Returns `auto auto`.
-    pub fn auto() -> Self {
-        GenericBackgroundSize::Explicit {
-            width: NonNegativeLengthPercentageOrAuto::auto(),
-            height: NonNegativeLengthPercentageOrAuto::auto(),
-        }
-    }
-}
-
 /// One of the keywords for `background-repeat`.
 #[derive(
     Clone,