Auto merge of servo#18352 - jdm:serialize-fun, r=emilio

Make serialization match Gecko in a few corner cases

This addresses the testcases from https://bugzilla.mozilla.org/show_bug.cgi?id=1345218. Gecko differs from the specification by doing the following:
* reusing longhands that have previously been serialized in order to serialize shorthands
* immediately breaking out of the shorthand loop for the current property as soon as a shorthand is successfully serialized

w3c/csswg-drafts#1774 is filed to track ways that the standard could be made more clear on these points.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [bug 1345218](https://bugzilla.mozilla.org/show_bug.cgi?id=1345218).
- [X] There are tests for these changes

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components/style/properties/declaration_block.rs

@@ -657,10 +657,6 @@ impl ToCss for PropertyDeclarationBlock {
 
                     if is_system_font {
                         for (longhand, importance) in self.declaration_importance_iter() {
-                            if already_serialized.contains(longhand.id()) {
-                                continue;
-                            }
-
                             if longhand.get_system().is_some() || longhand.is_default_line_height() {
                                 current_longhands.push(longhand);
                                 if found_system.is_none() {
@@ -673,10 +669,6 @@ impl ToCss for PropertyDeclarationBlock {
                         }
                     } else {
                         for (longhand, importance) in self.declaration_importance_iter() {
-                            if already_serialized.contains(longhand.id()) {
-                                continue;
-                            }
-
                             if longhand.id().is_longhand_of(shorthand) {
                                 current_longhands.push(longhand);
                                 if importance.important() {
@@ -771,6 +763,13 @@ impl ToCss for PropertyDeclarationBlock {
                         // Substep 9
                         already_serialized.insert(current_longhand.id());
                     }
+
+                    // FIXME(https://github.com/w3c/csswg-drafts/issues/1774)
+                    // The specification does not include an instruction to abort
+                    // the shorthand loop at this point, but doing so both matches
+                    // Gecko and makes sense since shorthands are checked in
+                    // preferred order.
+                    break;
                 }
             }
 

components/style/properties/properties.mako.rs

@@ -476,15 +476,28 @@ impl LonghandId {
         // algorithms and what not, I guess.
         <%
             longhand_to_shorthand_map = {}
+            num_sub_properties = {}
             for shorthand in data.shorthands:
+                num_sub_properties[shorthand.camel_case] = len(shorthand.sub_properties)
                 for sub_property in shorthand.sub_properties:
                     if sub_property.ident not in longhand_to_shorthand_map:
                         longhand_to_shorthand_map[sub_property.ident] = []
 
                     longhand_to_shorthand_map[sub_property.ident].append(shorthand.camel_case)
 
+            def preferred_order(x, y):
+                # Since we want properties in order from most subproperties to least,
+                # reverse the arguments to cmp from the expected order.
+                result = cmp(num_sub_properties.get(y, 0), num_sub_properties.get(x, 0))
+                if result:
+                    return result
+                # Fall back to lexicographic comparison.
+                return cmp(x, y)
+
+            # Sort the lists of shorthand properties according to preferred order:
+            # https://drafts.csswg.org/cssom/#concept-shorthands-preferred-order
             for shorthand_list in longhand_to_shorthand_map.itervalues():
-                shorthand_list.sort()
+                shorthand_list.sort(cmp=preferred_order)
         %>
 
         // based on lookup results for each longhand, create result arrays

tests/unit/style/properties/serialization.rs

@@ -341,6 +341,53 @@ mod shorthand_serialization {
     mod border_shorthands {
         use super::*;
 
+        #[test]
+        fn border_top_and_color() {
+            let mut properties = Vec::new();
+            properties.push(PropertyDeclaration::BorderTopWidth(BorderSideWidth::Length(Length::from_px(1.))));
+            properties.push(PropertyDeclaration::BorderTopStyle(BorderStyle::solid));
+            let c = Color::Numeric {
+                parsed: RGBA::new(255, 0, 0, 255),
+                authored: Some("green".to_string().into_boxed_str())
+            };
+            properties.push(PropertyDeclaration::BorderTopColor(c));
+            let c = Color::Numeric {
+                parsed: RGBA::new(0, 255, 0, 255),
+                authored: Some("red".to_string().into_boxed_str())
+            };
+            properties.push(PropertyDeclaration::BorderTopColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderBottomColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderLeftColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderRightColor(c.clone()));
+
+            let serialization = shorthand_properties_to_string(properties);
+            assert_eq!(serialization, "border-top: 1px solid red; border-color: red;");
+        }
+
+        #[test]
+        fn border_color_and_top() {
+            let mut properties = Vec::new();
+                let c = Color::Numeric {
+                parsed: RGBA::new(0, 255, 0, 255),
+                authored: Some("red".to_string().into_boxed_str())
+            };
+            properties.push(PropertyDeclaration::BorderTopColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderBottomColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderLeftColor(c.clone()));
+            properties.push(PropertyDeclaration::BorderRightColor(c.clone()));
+
+            properties.push(PropertyDeclaration::BorderTopWidth(BorderSideWidth::Length(Length::from_px(1.))));
+            properties.push(PropertyDeclaration::BorderTopStyle(BorderStyle::solid));
+            let c = Color::Numeric {
+                parsed: RGBA::new(255, 0, 0, 255),
+                authored: Some("green".to_string().into_boxed_str())
+            };
+            properties.push(PropertyDeclaration::BorderTopColor(c));
+
+            let serialization = shorthand_properties_to_string(properties);
+            assert_eq!(serialization, "border-color: green red red; border-top: 1px solid green;");
+        }
+
         // we can use border-top as a base to test out the different combinations
         // but afterwards, we only need to to one test per "directional border shorthand"