style: Simplify background-repeat.

This way we always serialize in the shortest form, and take less space.

This is useful because when serializing uncomputed values we'd like to compare
to the initial value to avoid serializing parts of a shorthand, but with the
existing implementation we would generate always a second keyword, which means
that we'll never match it.

This also matches Chrome and WebKit, incidentally, so I'm pretty confident the
behavior change when serializing specified style is web-compatible.

Differential Revision: https://phabricator.services.mozilla.com/D11941

components/style/values/specified/background.rs

@@ -9,7 +9,8 @@ use crate::values::generics::background::BackgroundSize as GenericBackgroundSize
 use crate::values::specified::length::NonNegativeLengthOrPercentageOrAuto;
 use cssparser::Parser;
 use selectors::parser::SelectorParseErrorKind;
-use style_traits::ParseError;
+use std::fmt::{self, Write};
+use style_traits::{CssWriter, ParseError, ToCss};
 
 /// A specified value for the `background-size` property.
 pub type BackgroundSize = GenericBackgroundSize<NonNegativeLengthOrPercentageOrAuto>;
@@ -56,6 +57,7 @@ impl BackgroundSize {
     ToCss,
 )]
 #[allow(missing_docs)]
+#[value_info(other_values = "repeat-x,repeat-y")]
 pub enum BackgroundRepeatKeyword {
     Repeat,
     Space,
@@ -63,24 +65,45 @@ pub enum BackgroundRepeatKeyword {
     NoRepeat,
 }
 
-/// The specified value for the `background-repeat` property.
+/// The value of the `background-repeat` property, with `repeat-x` / `repeat-y`
+/// represented as the combination of `no-repeat` and `repeat` in the opposite
+/// axes.
 ///
 /// https://drafts.csswg.org/css-backgrounds/#the-background-repeat
-#[derive(Clone, Copy, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToCss)]
-pub enum BackgroundRepeat {
-    /// `repeat-x`
-    RepeatX,
-    /// `repeat-y`
-    RepeatY,
-    /// `[repeat | space | round | no-repeat]{1,2}`
-    Keywords(BackgroundRepeatKeyword, Option<BackgroundRepeatKeyword>),
-}
+#[derive(Clone, Debug, MallocSizeOf, PartialEq, SpecifiedValueInfo, ToComputedValue)]
+pub struct BackgroundRepeat(pub BackgroundRepeatKeyword, pub BackgroundRepeatKeyword);
 
 impl BackgroundRepeat {
-    /// Returns the `repeat` value.
-    #[inline]
+    /// Returns the `repeat repeat` value.
     pub fn repeat() -> Self {
-        BackgroundRepeat::Keywords(BackgroundRepeatKeyword::Repeat, None)
+        BackgroundRepeat(
+            BackgroundRepeatKeyword::Repeat,
+            BackgroundRepeatKeyword::Repeat,
+        )
+    }
+}
+
+impl ToCss for BackgroundRepeat {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        match (self.0, self.1) {
+            (BackgroundRepeatKeyword::Repeat, BackgroundRepeatKeyword::NoRepeat) => {
+                dest.write_str("repeat-x")
+            },
+            (BackgroundRepeatKeyword::NoRepeat, BackgroundRepeatKeyword::Repeat) => {
+                dest.write_str("repeat-y")
+            },
+            (horizontal, vertical) => {
+                horizontal.to_css(dest)?;
+                if horizontal != vertical {
+                    dest.write_str(" ")?;
+                    vertical.to_css(dest)?;
+                }
+                Ok(())
+            },
+        }
     }
 }
 
@@ -92,8 +115,12 @@ impl Parse for BackgroundRepeat {
         let ident = input.expect_ident_cloned()?;
 
         match_ignore_ascii_case! { &ident,
-            "repeat-x" => return Ok(BackgroundRepeat::RepeatX),
-            "repeat-y" => return Ok(BackgroundRepeat::RepeatY),
+            "repeat-x" => {
+                return Ok(BackgroundRepeat(BackgroundRepeatKeyword::Repeat, BackgroundRepeatKeyword::NoRepeat));
+            },
+            "repeat-y" => {
+                return Ok(BackgroundRepeat(BackgroundRepeatKeyword::NoRepeat, BackgroundRepeatKeyword::Repeat));
+            },
             _ => {},
         }
 
@@ -107,6 +134,6 @@ impl Parse for BackgroundRepeat {
         };
 
         let vertical = input.try(BackgroundRepeatKeyword::parse).ok();
-        Ok(BackgroundRepeat::Keywords(horizontal, vertical))
+        Ok(BackgroundRepeat(horizontal, vertical.unwrap_or(horizontal)))
     }
 }