servo: Merge #14178 - Parse trait functions (from impowski:parse_trait_functions); r=Wafflespeanut

<!-- Please describe your changes on the following line: -->
Moved parse functions to a Parse trait in style. 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
- [X] These changes fix #14101 (github issue number if applicable).

<!-- Either: -->
- [x] There are tests for these changes OR
- [X] These changes do not require tests because new functionalities weren't added

<!-- 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: 6adae66f9c7ff58e0e31f3913471e575910a5861

servo/components/style/custom_properties.rs

@@ -8,6 +8,7 @@
 
 use Atom;
 use cssparser::{Delimiter, Parser, SourcePosition, Token, TokenSerializationType};
+use parser::Parse;
 use properties::DeclaredValue;
 use std::ascii::AsciiExt;
 use std::borrow::Cow;
@@ -111,15 +112,17 @@ impl ComputedValue {
     }
 }
 
-pub fn parse(input: &mut Parser) -> Result<SpecifiedValue, ()> {
-    let mut references = Some(HashSet::new());
-    let (first, css, last) = try!(parse_self_contained_declaration_value(input, &mut references));
-    Ok(SpecifiedValue {
-        css: css.into_owned(),
-        first_token_type: first,
-        last_token_type: last,
-        references: references.unwrap(),
-    })
+impl Parse for SpecifiedValue {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        let mut references = Some(HashSet::new());
+        let (first, css, last) = try!(parse_self_contained_declaration_value(input, &mut references));
+        Ok(SpecifiedValue {
+            css: css.into_owned(),
+            first_token_type: first,
+            last_token_type: last,
+            references: references.unwrap(),
+        })
+    }
 }
 
 /// Parse the value of a non-custom property that contains `var()` references.

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

@@ -67,7 +67,7 @@
 
             pub mod single_value {
                 use cssparser::Parser;
-                use parser::{ParserContext, ParserContextExtraData};
+                use parser::{Parse, ParserContext, ParserContextExtraData};
                 use properties::{CSSWideKeyword, DeclaredValue, Shorthand};
                 use values::computed::{Context, ToComputedValue};
                 use values::{computed, specified};
@@ -175,13 +175,12 @@
         #![allow(unused_imports)]
         % if not property.derived_from:
             use cssparser::Parser;
-            use parser::{ParserContext, ParserContextExtraData};
+            use parser::{Parse, ParserContext, ParserContextExtraData};
             use properties::{CSSWideKeyword, DeclaredValue, Shorthand};
         % endif
         use values::{Auto, Either, None_, Normal};
         use cascade_info::CascadeInfo;
         use error_reporting::ParseErrorReporter;
-        use parser::Parse;
         use properties::longhands;
         use properties::property_bit_field::PropertyBitField;
         use properties::{ComputedValues, PropertyDeclaration};

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

@@ -15,7 +15,6 @@ ${helpers.predefined_type("opacity",
 <%helpers:vector_longhand name="box-shadow" allow_empty="True" animatable="True">
     use cssparser;
     use std::fmt;
-    use parser::Parse;
     use style_traits::ToCss;
     use values::HasViewportPercentage;
 
@@ -355,7 +354,6 @@ ${helpers.predefined_type("opacity",
 
     pub fn parse(_context: &ParserContext, input: &mut Parser) -> Result<SpecifiedValue, ()> {
         use app_units::Au;
-        use parser::Parse;
         use std::ascii::AsciiExt;
         use values::specified::Length;
 
@@ -677,7 +675,6 @@ pub struct OriginParseResult {
 }
 
 pub fn parse_origin(_: &ParserContext, input: &mut Parser) -> Result<OriginParseResult,()> {
-    use parser::Parse;
     use values::specified::{LengthOrPercentage, Percentage};
     let (mut horizontal, mut vertical, mut depth) = (None, None, None);
     loop {

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

@@ -496,6 +496,7 @@ ${helpers.single_keyword("font-variant-position",
 
     pub mod computed_value {
         use cssparser::Parser;
+        use parser::Parse;
         use std::fmt;
         use style_traits::ToCss;
 
@@ -542,10 +543,10 @@ ${helpers.single_keyword("font-variant-position",
             }
         }
 
-        impl FeatureTagValue {
+        impl Parse for FeatureTagValue {
             /// https://www.w3.org/TR/css-fonts-3/#propdef-font-feature-settings
             /// <string> [ on | off | <integer> ]
-            pub fn parse(input: &mut Parser) -> Result<FeatureTagValue, ()> {
+            fn parse(input: &mut Parser) -> Result<Self, ()> {
                 let tag = try!(input.expect_string());
 
                 // allowed strings of length 4 containing chars: <U+20, U+7E>

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

@@ -27,7 +27,7 @@ use euclid::size::Size2D;
 use computed_values;
 #[cfg(feature = "servo")] use logical_geometry::{LogicalMargin, PhysicalSide};
 use logical_geometry::WritingMode;
-use parser::{ParserContext, ParserContextExtraData};
+use parser::{Parse, ParserContext, ParserContextExtraData};
 use style_traits::ToCss;
 use stylesheets::Origin;
 #[cfg(feature = "servo")] use values::Either;
@@ -49,7 +49,7 @@ pub mod declaration_block;
 
 pub mod longhands {
     use cssparser::Parser;
-    use parser::ParserContext;
+    use parser::{Parse, ParserContext};
     use values::specified;
 
     <%include file="/longhand/background.mako.rs" />
@@ -79,7 +79,7 @@ pub mod longhands {
 
 pub mod shorthands {
     use cssparser::Parser;
-    use parser::ParserContext;
+    use parser::{Parse, ParserContext};
     use values::specified;
 
     pub fn parse_four_sides<F, T>(input: &mut Parser, parse_one: F) -> Result<(T, T, T, T), ()>
@@ -344,8 +344,8 @@ pub enum CSSWideKeyword {
     UnsetKeyword,
 }
 
-impl CSSWideKeyword {
-    pub fn parse(input: &mut Parser) -> Result<CSSWideKeyword, ()> {
+impl Parse for CSSWideKeyword {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         match_ignore_ascii_case! { try!(input.expect_ident()),
             "initial" => Ok(CSSWideKeyword::InitialKeyword),
             "inherit" => Ok(CSSWideKeyword::InheritKeyword),
@@ -736,7 +736,7 @@ impl PropertyDeclaration {
                 Ok(CSSWideKeyword::UnsetKeyword) |  // Custom properties are alawys inherited
                 Ok(CSSWideKeyword::InheritKeyword) => DeclaredValue::Inherit,
                 Ok(CSSWideKeyword::InitialKeyword) => DeclaredValue::Initial,
-                Err(()) => match ::custom_properties::parse(input) {
+                Err(()) => match ::custom_properties::SpecifiedValue::parse(input) {
                     Ok(value) => DeclaredValue::Value(value),
                     Err(()) => return PropertyDeclarationParseResult::InvalidValue,
                 }

servo/components/style/properties/shorthand/border.mako.rs

@@ -149,6 +149,7 @@ pub fn parse_border(context: &ParserContext, input: &mut Parser)
      for corner in ['top-left', 'top-right', 'bottom-right', 'bottom-left']
 )}">
     use values::specified::basic_shape::BorderRadius;
+    use parser::Parse;
 
     pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
         let _ignored = context;

servo/components/style/properties/shorthand/outline.mako.rs

@@ -7,6 +7,7 @@
 <%helpers:shorthand name="outline" sub_properties="outline-color outline-style outline-width">
     use properties::longhands::outline_width;
     use values::specified;
+    use parser::Parse;
 
     pub fn parse_value(context: &ParserContext, input: &mut Parser) -> Result<Longhands, ()> {
         let _unused = context;

servo/components/style/properties/shorthand/position.mako.rs

@@ -56,6 +56,7 @@
 
 // https://drafts.csswg.org/css-flexbox/#flex-property
 <%helpers:shorthand name="flex" sub_properties="flex-grow flex-shrink flex-basis">
+    use parser::Parse;
     use app_units::Au;
     use values::specified::{Number, Length, LengthOrPercentageOrAutoOrContent};
 

servo/components/style/values/mod.rs

@@ -7,7 +7,6 @@
 //! [values]: https://drafts.csswg.org/css-values/
 
 pub use cssparser::{RGBA, Parser};
-
 use parser::Parse;
 use std::fmt::{self, Debug};
 use style_traits::ToCss;

servo/components/style/values/specified/basic_shape.rs

@@ -213,14 +213,6 @@ pub struct InsetRect {
 }
 
 impl InsetRect {
-    pub fn parse(input: &mut Parser) -> Result<InsetRect, ()> {
-        match_ignore_ascii_case! { try!(input.expect_function()),
-            "inset" => {
-                Ok(try!(input.parse_nested_block(InsetRect::parse_function_arguments)))
-            },
-            _ => Err(())
-        }
-    }
     pub fn parse_function_arguments(input: &mut Parser) -> Result<InsetRect, ()> {
         let (t, r, b, l) = try!(parse_four_sides(input, LengthOrPercentage::parse));
         let mut rect = InsetRect {
@@ -237,6 +229,17 @@ impl InsetRect {
     }
 }
 
+impl Parse for InsetRect {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        match_ignore_ascii_case! { try!(input.expect_function()),
+                                   "inset" => {
+                                       Ok(try!(input.parse_nested_block(InsetRect::parse_function_arguments)))
+                                   },
+                                   _ => Err(())
+        }
+    }
+}
+
 impl ToCss for InsetRect {
     // XXXManishearth again, we should try to reduce the number of values printed here
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
@@ -374,14 +377,6 @@ pub struct Circle {
 }
 
 impl Circle {
-    pub fn parse(input: &mut Parser) -> Result<Circle, ()> {
-        match_ignore_ascii_case! { try!(input.expect_function()),
-            "circle" => {
-                Ok(try!(input.parse_nested_block(Circle::parse_function_arguments)))
-            },
-            _ => Err(())
-        }
-    }
     pub fn parse_function_arguments(input: &mut Parser) -> Result<Circle, ()> {
         let radius = input.try(ShapeRadius::parse).ok().unwrap_or_else(Default::default);
         let position = if let Ok(_) = input.try(|input| input.expect_ident_matching("at")) {
@@ -402,6 +397,17 @@ impl Circle {
     }
 }
 
+impl Parse for Circle {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        match_ignore_ascii_case! { try!(input.expect_function()),
+                                   "circle" => {
+                                       Ok(try!(input.parse_nested_block(Circle::parse_function_arguments)))
+                                   },
+                                   _ => Err(())
+        }
+    }
+}
+
 impl ToCss for Circle {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         try!(dest.write_str("circle("));
@@ -446,14 +452,6 @@ pub struct Ellipse {
 
 
 impl Ellipse {
-    pub fn parse(input: &mut Parser) -> Result<Ellipse, ()> {
-        match_ignore_ascii_case! { try!(input.expect_function()),
-            "ellipse" => {
-                Ok(try!(input.parse_nested_block(Ellipse::parse_function_arguments)))
-            },
-            _ => Err(())
-        }
-    }
     pub fn parse_function_arguments(input: &mut Parser) -> Result<Ellipse, ()> {
         let (a, b) = input.try(|input| -> Result<_, ()> {
             Ok((try!(ShapeRadius::parse(input)), try!(ShapeRadius::parse(input))))
@@ -477,6 +475,17 @@ impl Ellipse {
     }
 }
 
+impl Parse for Ellipse {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        match_ignore_ascii_case! { try!(input.expect_function()),
+                                   "ellipse" => {
+                                       Ok(try!(input.parse_nested_block(Ellipse::parse_function_arguments)))
+                                   },
+                                   _ => Err(())
+        }
+    }
+}
+
 impl ToCss for Ellipse {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         try!(dest.write_str("ellipse("));
@@ -524,14 +533,6 @@ pub struct Polygon {
 }
 
 impl Polygon {
-    pub fn parse(input: &mut Parser) -> Result<Polygon, ()> {
-        match_ignore_ascii_case! { try!(input.expect_function()),
-            "polygon" => {
-                Ok(try!(input.parse_nested_block(Polygon::parse_function_arguments)))
-            },
-            _ => Err(())
-        }
-    }
     pub fn parse_function_arguments(input: &mut Parser) -> Result<Polygon, ()> {
         let fill = input.try(|input| {
             let fill = FillRule::parse(input);
@@ -550,6 +551,17 @@ impl Polygon {
     }
 }
 
+impl Parse for Polygon {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        match_ignore_ascii_case! { try!(input.expect_function()),
+                                   "polygon" => {
+                                       Ok(try!(input.parse_nested_block(Polygon::parse_function_arguments)))
+                                   },
+                                   _ => Err(())
+        }
+    }
+}
+
 impl ToCss for Polygon {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         try!(dest.write_str("polygon("));
@@ -616,8 +628,8 @@ impl Default for ShapeRadius {
     }
 }
 
-impl ShapeRadius {
-    pub fn parse(input: &mut Parser) -> Result<ShapeRadius, ()> {
+impl Parse for ShapeRadius {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         input.try(LengthOrPercentage::parse).map(ShapeRadius::Length)
                                             .or_else(|_| {
             match_ignore_ascii_case! { try!(input.expect_ident()),
@@ -703,8 +715,8 @@ impl ToCss for BorderRadius {
     }
 }
 
-impl BorderRadius {
-    pub fn parse(input: &mut Parser) -> Result<BorderRadius, ()> {
+impl Parse for BorderRadius {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         let widths = try!(parse_one_set_of_border_values(input));
         let heights = if input.try(|input| input.expect_delim('/')).is_ok() {
             try!(parse_one_set_of_border_values(input))
@@ -781,8 +793,8 @@ pub enum FillRule {
 
 impl ComputedValueAsSpecified for FillRule {}
 
-impl FillRule {
-    pub fn parse(input: &mut Parser) -> Result<FillRule, ()> {
+impl Parse for FillRule {
+    fn parse(input: &mut Parser) -> Result<FillRule, ()> {
         match_ignore_ascii_case! { try!(input.expect_ident()),
             "nonzero" => Ok(FillRule::NonZero),
             "evenodd" => Ok(FillRule::EvenOdd),

servo/components/style/values/specified/length.rs

@@ -840,15 +840,18 @@ impl LengthOrPercentageOrAuto {
         }
     }
     #[inline]
-    pub fn parse(input: &mut Parser) -> Result<LengthOrPercentageOrAuto, ()> {
-        LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::All)
-    }
-    #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentageOrAuto, ()> {
         LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::NonNegative)
     }
 }
 
+impl Parse for LengthOrPercentageOrAuto {
+    #[inline]
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        LengthOrPercentageOrAuto::parse_internal(input, AllowedNumericType::All)
+    }
+}
+
 #[derive(Clone, PartialEq, Copy, Debug)]
 #[cfg_attr(feature = "servo", derive(HeapSizeOf))]
 pub enum LengthOrPercentageOrNone {
@@ -899,15 +902,18 @@ impl LengthOrPercentageOrNone {
         }
     }
     #[inline]
-    pub fn parse(input: &mut Parser) -> Result<LengthOrPercentageOrNone, ()> {
-        LengthOrPercentageOrNone::parse_internal(input, AllowedNumericType::All)
-    }
-    #[inline]
     pub fn parse_non_negative(input: &mut Parser) -> Result<LengthOrPercentageOrNone, ()> {
         LengthOrPercentageOrNone::parse_internal(input, AllowedNumericType::NonNegative)
     }
 }
 
+impl Parse for LengthOrPercentageOrNone {
+    #[inline]
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        LengthOrPercentageOrNone::parse_internal(input, AllowedNumericType::All)
+    }
+}
+
 pub type LengthOrNone = Either<Length, None_>;
 
 impl LengthOrNone {
@@ -949,8 +955,8 @@ impl ToCss for LengthOrPercentageOrAutoOrContent {
     }
 }
 
-impl LengthOrPercentageOrAutoOrContent {
-    pub fn parse(input: &mut Parser) -> Result<LengthOrPercentageOrAutoOrContent, ()> {
+impl Parse for LengthOrPercentageOrAutoOrContent {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         let context = AllowedNumericType::NonNegative;
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) if context.is_ok(value.value) =>

servo/components/style/values/specified/mod.rs

@@ -5,7 +5,7 @@
 use app_units::Au;
 use cssparser::{self, Parser, Token};
 use euclid::size::Size2D;
-use parser::ParserContext;
+use parser::{Parse, ParserContext};
 use self::url::SpecifiedUrl;
 use std::ascii::AsciiExt;
 use std::f32::consts::PI;
@@ -36,8 +36,9 @@ pub struct CSSColor {
     pub parsed: cssparser::Color,
     pub authored: Option<String>,
 }
-impl CSSColor {
-    pub fn parse(input: &mut Parser) -> Result<CSSColor, ()> {
+
+impl Parse for CSSColor {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         let start_position = input.position();
         let authored = match input.next() {
             Ok(Token::Ident(s)) => Some(s.into_owned()),
@@ -192,9 +193,11 @@ impl BorderRadiusSize {
     pub fn circle(radius: LengthOrPercentage) -> BorderRadiusSize {
         BorderRadiusSize(Size2D::new(radius, radius))
     }
+}
 
+impl Parse for BorderRadiusSize {
     #[inline]
-    pub fn parse(input: &mut Parser) -> Result<BorderRadiusSize, ()> {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         let first = try!(LengthOrPercentage::parse_non_negative(input));
         let second = input.try(LengthOrPercentage::parse_non_negative).unwrap_or(first);
         Ok(BorderRadiusSize(Size2D::new(first, second)))
@@ -236,9 +239,9 @@ const RAD_PER_DEG: CSSFloat = PI / 180.0;
 const RAD_PER_GRAD: CSSFloat = PI / 200.0;
 const RAD_PER_TURN: CSSFloat = PI * 2.0;
 
-impl Angle {
+impl Parse for Angle {
     /// Parses an angle according to CSS-VALUES § 6.1.
-    pub fn parse(input: &mut Parser) -> Result<Angle, ()> {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         match try!(input.next()) {
             Token::Dimension(ref value, ref unit) => Angle::parse_dimension(value.value, unit),
             Token::Number(ref value) if value.value == 0. => Ok(Angle(0.)),
@@ -248,7 +251,9 @@ impl Angle {
             _ => Err(())
         }
     }
+}
 
+impl Angle {
     pub fn parse_dimension(value: CSSFloat, unit: &str) -> Result<Angle, ()> {
         match_ignore_ascii_case! { unit,
             "deg" => Ok(Angle(value * RAD_PER_DEG)),
@@ -399,8 +404,12 @@ impl Time {
             Err(())
         }
     }
+}
 
-    pub fn parse(input: &mut Parser) -> Result<Time, ()> {
+impl ComputedValueAsSpecified for Time {}
+
+impl Parse for Time {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         match input.next() {
             Ok(Token::Dimension(ref value, ref unit)) => {
                 Time::parse_dimension(value.value, &unit)
@@ -413,8 +422,6 @@ impl Time {
     }
 }
 
-impl ComputedValueAsSpecified for Time {}
-
 impl ToCss for Time {
     fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
         write!(dest, "{}s", self.0)
@@ -427,11 +434,13 @@ pub struct Number(pub CSSFloat);
 
 impl NoViewportPercentage for Number {}
 
-impl Number {
-    pub fn parse(input: &mut Parser) -> Result<Number, ()> {
+impl Parse for Number {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         parse_number(input).map(Number)
     }
+}
 
+impl Number {
     fn parse_with_minimum(input: &mut Parser, min: CSSFloat) -> Result<Number, ()> {
         match parse_number(input) {
             Ok(value) if value < min => Err(()),
@@ -472,8 +481,8 @@ pub struct Opacity(pub CSSFloat);
 
 impl NoViewportPercentage for Opacity {}
 
-impl Opacity {
-    pub fn parse(input: &mut Parser) -> Result<Opacity, ()> {
+impl Parse for Opacity {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         parse_number(input).map(Opacity)
     }
 }

servo/components/style/values/specified/position.rs

@@ -169,7 +169,18 @@ impl Position {
         })
     }
 
-    pub fn parse(input: &mut Parser) -> Result<Position, ()> {
+    pub fn center() -> Position {
+        Position {
+            horiz_keyword: Some(Keyword::Center),
+            horiz_position: None,
+            vert_keyword: Some(Keyword::Center),
+            vert_position: None,
+        }
+    }
+}
+
+impl Parse for Position {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
         let first = try!(PositionComponent::parse(input));
         let second = input.try(PositionComponent::parse)
             .unwrap_or(PositionComponent::Keyword(Keyword::Center));
@@ -216,15 +227,6 @@ impl Position {
             }
         }
     }
-
-    pub fn center() -> Position {
-        Position {
-            horiz_keyword: Some(Keyword::Center),
-            horiz_position: None,
-            vert_keyword: Some(Keyword::Center),
-            vert_position: None,
-        }
-    }
 }
 
 impl Keyword {
@@ -362,25 +364,6 @@ impl HasViewportPercentage for PositionComponent {
 }
 
 impl PositionComponent {
-    pub fn parse(input: &mut Parser) -> Result<PositionComponent, ()> {
-        input.try(LengthOrPercentage::parse)
-        .map(PositionComponent::Length)
-        .or_else(|()| {
-            match try!(input.next()) {
-                Token::Ident(value) => {
-                    match_ignore_ascii_case! { value,
-                        "center" => Ok(PositionComponent::Keyword(Keyword::Center)),
-                        "left" => Ok(PositionComponent::Keyword(Keyword::Left)),
-                        "right" => Ok(PositionComponent::Keyword(Keyword::Right)),
-                        "top" => Ok(PositionComponent::Keyword(Keyword::Top)),
-                        "bottom" => Ok(PositionComponent::Keyword(Keyword::Bottom)),
-                        _ => Err(())
-                    }
-                },
-                _ => Err(())
-            }
-        })
-    }
     #[inline]
     pub fn to_length_or_percentage(self) -> LengthOrPercentage {
         match self {
@@ -389,3 +372,25 @@ impl PositionComponent {
         }
     }
 }
+
+impl Parse for PositionComponent {
+    fn parse(input: &mut Parser) -> Result<Self, ()> {
+        input.try(LengthOrPercentage::parse)
+            .map(PositionComponent::Length)
+            .or_else(|()| {
+                match try!(input.next()) {
+                    Token::Ident(value) => {
+                        match_ignore_ascii_case! { value,
+                                                   "center" => Ok(PositionComponent::Keyword(Keyword::Center)),
+                                                   "left" => Ok(PositionComponent::Keyword(Keyword::Left)),
+                                                   "right" => Ok(PositionComponent::Keyword(Keyword::Right)),
+                                                   "top" => Ok(PositionComponent::Keyword(Keyword::Top)),
+                                                   "bottom" => Ok(PositionComponent::Keyword(Keyword::Bottom)),
+                                                   _ => Err(())
+                        }
+                    },
+                    _ => Err(())
+                }
+            })
+    }
+}

servo/tests/unit/style/parsing/position.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use parsing::parse;
+use style::parser::Parse;
 use style::values::specified::position::*;
 use style_traits::ToCss;