servo: Merge #13937 - Implements parser/serialization for font-synthesis (from iamrohit7:font-synthesis); r=emilio

<!-- 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
- [X] These changes fix #13876 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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

Source-Repo: https://github.com/servo/servo
Source-Revision: 0193f987f6b8cdce02c297529f823e403da49b20

servo/components/style/properties/gecko.mako.rs

@@ -765,7 +765,7 @@ fn static_assert() {
 </%self:impl_trait>
 
 <%self:impl_trait style_struct_name="Font"
-    skip_longhands="font-family font-size font-weight"
+    skip_longhands="font-family font-size font-weight font-synthesis"
     skip_additionals="*">
 
     pub fn set_font_family(&mut self, v: longhands::font_family::computed_value::T) {
@@ -828,6 +828,22 @@ fn static_assert() {
     // This is used for PartialEq, which we don't implement for gecko style structs.
     pub fn compute_font_hash(&mut self) {}
 
+    pub fn set_font_synthesis(&mut self, v: longhands::font_synthesis::computed_value::T) {
+        use gecko_bindings::structs::{NS_FONT_SYNTHESIS_WEIGHT, NS_FONT_SYNTHESIS_STYLE};
+
+        self.gecko.mFont.synthesis = 0;
+        if v.weight {
+            self.gecko.mFont.synthesis |= NS_FONT_SYNTHESIS_WEIGHT as u8;
+        }
+        if v.style {
+            self.gecko.mFont.synthesis |= NS_FONT_SYNTHESIS_STYLE as u8;
+        }
+    }
+
+    pub fn copy_font_synthesis_from(&mut self, other: &Self) {
+        self.gecko.mFont.synthesis = other.gecko.mFont.synthesis;
+    }
+
 </%self:impl_trait>
 
 <% skip_box_longhands= """display overflow-y vertical-align

servo/components/style/properties/longhand/font.mako.rs

@@ -344,6 +344,69 @@ ${helpers.single_keyword("font-variant",
     }
 </%helpers:longhand>
 
+<%helpers:longhand products="gecko" name="font-synthesis" animatable="False">
+    use cssparser::ToCss;
+    use std::fmt;
+    use values::LocalToCss;
+    use values::computed::ComputedValueAsSpecified;
+    use values::NoViewportPercentage;
+
+    impl ComputedValueAsSpecified for SpecifiedValue {}
+    impl NoViewportPercentage for SpecifiedValue {}
+
+    pub mod computed_value {
+        pub use super::SpecifiedValue as T;
+    }
+
+    #[derive(Debug, Clone, PartialEq)]
+    #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
+    pub struct SpecifiedValue {
+        pub weight: bool,
+        pub style: bool,
+    }
+
+    impl ToCss for computed_value::T {
+        fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+            if self.weight && self.style {
+                dest.write_str("weight style")
+            } else if self.style {
+                dest.write_str("style")
+            } else if self.weight {
+                dest.write_str("weight")
+            } else {
+                dest.write_str("none")
+            }
+        }
+    }
+
+    #[inline]
+    pub fn get_initial_value() -> computed_value::T {
+        SpecifiedValue { weight: true, style: true }
+    }
+
+    pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
+        let mut result = SpecifiedValue { weight: false, style: false };
+        match_ignore_ascii_case! {try!(input.expect_ident()),
+            "none" => Ok(result),
+            "weight" => {
+                result.weight = true;
+                if input.try(|input| input.expect_ident_matching("style")).is_ok() {
+                    result.style = true;
+                }
+                Ok(result)
+            },
+            "style" => {
+                result.style = true;
+                if input.try(|input| input.expect_ident_matching("weight")).is_ok() {
+                    result.weight = true;
+                }
+                Ok(result)
+            },
+            _ => Err(())
+        }
+    }
+</%helpers:longhand>
+
 // FIXME: This prop should be animatable
 ${helpers.single_keyword("font-stretch",
                          "normal ultra-condensed extra-condensed condensed \