Auto merge of #14104 - iamrohit7:scroll-snap-type, r=Manishearth,waffles

Adds scroll-snap-type shorthand property

<!-- Please describe your changes on the following line: -->
Follow up to #14017

---
r=Manishearth
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors

<!-- Either: -->
- [X] There are tests for these changes `scroll_snap_type::should_serialize_to_single_value_if_sub_types_are_equal`, `scroll_snap_type::should_serialize_to_empty_string_if_sub_types_not_equal`

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

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---
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components/style/properties/declaration_block.rs

@@ -214,16 +214,13 @@ impl PropertyDeclarationBlock {
                 if !self.declarations.iter().all(|decl| decl.0.shorthands().contains(&shorthand)) {
                     return Err(fmt::Error)
                 }
-                let success = try!(shorthand.serialize_shorthand_to_buffer(
+                let iter = self.declarations.iter().map(get_declaration as fn(_) -> _);
-                    dest,
+                match shorthand.get_shorthand_appendable_value(iter) {
-                    self.declarations.iter()
+                    Some(AppendableValue::Css(css)) => dest.write_str(css),
-                                     .map(get_declaration as fn(_) -> _),
+                    Some(AppendableValue::DeclarationsForShorthand(_, decls)) => {
-                    &mut true)
+                        shorthand.longhands_to_css(decls, dest)
-                );
+                    }
-                if success {
+                    _ => Ok(())
-                    Ok(())
-                } else {
-                    Err(fmt::Error)
                 }
             }
         }

components/style/properties/gecko.mako.rs

@@ -978,7 +978,8 @@ fn static_assert() {
 
 <% skip_box_longhands= """display overflow-y vertical-align
                           -moz-binding page-break-before page-break-after
-                          scroll-snap-points-x scroll-snap-points-y transform""" %>
+                          scroll-snap-points-x scroll-snap-points-y transform
+                          scroll-snap-type-y""" %>
 <%self:impl_trait style_struct_name="Box" skip_longhands="${skip_box_longhands}">
 
     // We manually-implement the |display| property until we get general
@@ -1220,6 +1221,11 @@ fn static_assert() {
     pub fn copy_transform_from(&mut self, other: &Self) {
         unsafe { self.gecko.mSpecifiedTransform.set(&other.gecko.mSpecifiedTransform); }
     }
+
+    <% scroll_snap_type_keyword = Keyword("scroll-snap-type", "none mandatory proximity") %>
+
+    ${impl_keyword('scroll_snap_type_y', 'mScrollSnapTypeY', scroll_snap_type_keyword, need_clone=False)}
+
 </%self:impl_trait>
 
 <%def name="simple_image_array_property(name, shorthand, field_name)">

components/style/properties/longhand/box.mako.rs

@@ -1474,11 +1474,12 @@ ${helpers.single_keyword("scroll-snap-type-x",
                          animatable=False)}
 
 // Non-standard: https://developer.mozilla.org/en-US/docs/Web/CSS/scroll-snap-type-y
-${helpers.single_keyword("scroll-snap-type-y",
+<%helpers:longhand products="gecko" name="scroll-snap-type-y" animatable="False">
-                         "none mandatory proximity",
+    pub use super::scroll_snap_type_x::SpecifiedValue;
-                         products="gecko",
+    pub use super::scroll_snap_type_x::computed_value;
-                         gecko_constant_prefix="NS_STYLE_SCROLL_SNAP_TYPE",
+    pub use super::scroll_snap_type_x::get_initial_value;
-                         animatable=False)}
+    pub use super::scroll_snap_type_x::parse;
+</%helpers:longhand>
 
 // Compositing and Blending Level 1
 // http://www.w3.org/TR/compositing-1/

components/style/properties/shorthand/box.mako.rs

@@ -297,3 +297,37 @@ macro_rules! try_parse_one {
         }
     }
 </%helpers:shorthand>
+
+<%helpers:shorthand name="scroll-snap-type" products="gecko"
+                    sub_properties="scroll-snap-type-x scroll-snap-type-y">
+    use properties::longhands::scroll_snap_type_x;
+
+    pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
+        let result = try!(scroll_snap_type_x::parse(context, input));
+        Ok(Longhands {
+            scroll_snap_type_x: Some(result),
+            scroll_snap_type_y: Some(result),
+        })
+    }
+
+    impl<'a> LonghandsToSerialize<'a>  {
+        // Serializes into the single keyword value if both scroll-snap-type and scroll-snap-type-y are same.
+        // Otherwise into an empty string. This is done to match Gecko's behaviour.
+        fn to_css_declared<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            let x_and_y_equal = match (self.scroll_snap_type_x, self.scroll_snap_type_y) {
+                (&DeclaredValue::Value(ref x_value), &DeclaredValue::Value(ref y_value)) => {
+                    *x_value == *y_value
+                },
+                (&DeclaredValue::Initial, &DeclaredValue::Initial) => true,
+                (&DeclaredValue::Inherit, &DeclaredValue::Inherit) => true,
+                (x, y) => { *x == *y },
+            };
+
+            if x_and_y_equal {
+                self.scroll_snap_type_x.to_css(dest)
+            } else {
+                Ok(())
+            }
+        }
+    }
+</%helpers:shorthand>

tests/unit/style/properties/serialization.rs

@@ -1006,4 +1006,53 @@ mod shorthand_serialization {
             );
         }
     }
+
+    mod scroll_snap_type {
+        pub use super::*;
+        use style::properties::longhands::scroll_snap_type_x::computed_value::T as ScrollSnapTypeXValue;
+
+        #[test]
+        fn should_serialize_to_empty_string_if_sub_types_not_equal() {
+            let declarations = vec![
+                (PropertyDeclaration::ScrollSnapTypeX(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal),
+                (PropertyDeclaration::ScrollSnapTypeY(DeclaredValue::Value(ScrollSnapTypeXValue::none)),
+                Importance::Normal)
+            ];
+
+            let block = PropertyDeclarationBlock {
+                declarations: declarations,
+                important_count: 0
+            };
+
+            let mut s = String::new();
+
+            let x = block.single_value_to_css("scroll-snap-type", &mut s);
+
+            assert_eq!(x.is_ok(), true);
+            assert_eq!(s, "");
+        }
+
+        #[test]
+        fn should_serialize_to_single_value_if_sub_types_are_equal() {
+            let declarations = vec![
+                (PropertyDeclaration::ScrollSnapTypeX(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal),
+                (PropertyDeclaration::ScrollSnapTypeY(DeclaredValue::Value(ScrollSnapTypeXValue::mandatory)),
+                Importance::Normal)
+            ];
+
+            let block = PropertyDeclarationBlock {
+                declarations: declarations,
+                important_count: 0
+            };
+
+            let mut s = String::new();
+
+            let x = block.single_value_to_css("scroll-snap-type", &mut s);
+
+            assert_eq!(x.is_ok(), true);
+            assert_eq!(s, "mandatory");
+        }
+    }
 }