Auto merge of #18209 - jdm:devirtualize, r=mbrubeck

Devirtualize CSS error reporting.

This removes a trait object from the path of reporting a CSS error.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/18209)
<!-- Reviewable:end -->

components/style/stylesheets/font_feature_values_rule.rs

@@ -10,8 +10,8 @@ use Atom;
 use computed_values::font_family::FamilyName;
 use cssparser::{AtRuleParser, AtRuleType, BasicParseError, DeclarationListParser, DeclarationParser, Parser};
 use cssparser::{CowRcStr, RuleListParser, SourceLocation, QualifiedRuleParser, Token, serialize_identifier};
-use error_reporting::ContextualParseError;
-use parser::{ParserContext, Parse};
+use error_reporting::{ContextualParseError, ParseErrorReporter};
+use parser::{ParserContext, ParserErrorContext, Parse};
 use selectors::parser::SelectorParseError;
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt;
@@ -210,20 +210,26 @@ macro_rules! font_feature_values_blocks {
             }
 
             /// Parses a `FontFeatureValuesRule`.
-            pub fn parse(context: &ParserContext, input: &mut Parser,
-                         family_names: Vec<FamilyName>, location: SourceLocation)
-                         -> FontFeatureValuesRule {
+            pub fn parse<R>(context: &ParserContext,
+                            error_context: &ParserErrorContext<R>,
+                            input: &mut Parser,
+                            family_names: Vec<FamilyName>,
+                            location: SourceLocation)
+                            -> FontFeatureValuesRule
+                where R: ParseErrorReporter
+            {
                 let mut rule = FontFeatureValuesRule::new(family_names, location);
 
                 {
                     let mut iter = RuleListParser::new_for_nested_rule(input, FontFeatureValuesRuleParser {
                         context: context,
+                        error_context: error_context,
                         rule: &mut rule,
                     });
                     while let Some(result) = iter.next() {
                         if let Err(err) = result {
                             let error = ContextualParseError::UnsupportedRule(err.slice, err.error);
-                            context.log_css_error(err.location, error);
+                            context.log_css_error(error_context, err.location, error);
                         }
                     }
                 }
@@ -293,19 +299,20 @@ macro_rules! font_feature_values_blocks {
         /// }
         /// <feature-type> = @stylistic | @historical-forms | @styleset |
         /// @character-variant | @swash | @ornaments | @annotation
-        struct FontFeatureValuesRuleParser<'a> {
+        struct FontFeatureValuesRuleParser<'a, R: 'a> {
             context: &'a ParserContext<'a>,
+            error_context: &'a ParserErrorContext<'a, R>,
             rule: &'a mut FontFeatureValuesRule,
         }
 
         /// Default methods reject all qualified rules.
-        impl<'a, 'i> QualifiedRuleParser<'i> for FontFeatureValuesRuleParser<'a> {
+        impl<'a, 'i, R: ParseErrorReporter> QualifiedRuleParser<'i> for FontFeatureValuesRuleParser<'a, R> {
             type Prelude = ();
             type QualifiedRule = ();
             type Error = SelectorParseError<'i, StyleParseError<'i>>;
         }
 
-        impl<'a, 'i> AtRuleParser<'i> for FontFeatureValuesRuleParser<'a> {
+        impl<'a, 'i, R: ParseErrorReporter> AtRuleParser<'i> for FontFeatureValuesRuleParser<'a, R> {
             type Prelude = BlockType;
             type AtRule = ();
             type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -341,7 +348,7 @@ macro_rules! font_feature_values_blocks {
                                 if let Err(err) = declaration {
                                     let error = ContextualParseError::UnsupportedKeyframePropertyDeclaration(
                                         err.slice, err.error);
-                                    self.context.log_css_error(err.location, error);
+                                    self.context.log_css_error(self.error_context, err.location, error);
                                 }
                             }
                         },

components/style/stylesheets/keyframes_rule.rs

@@ -6,8 +6,8 @@
 
 use cssparser::{AtRuleParser, Parser, QualifiedRuleParser, RuleListParser, ParserInput, CowRcStr};
 use cssparser::{DeclarationListParser, DeclarationParser, parse_one_rule, SourceLocation};
-use error_reporting::{NullReporter, ContextualParseError};
-use parser::ParserContext;
+use error_reporting::{NullReporter, ContextualParseError, ParseErrorReporter};
+use parser::{ParserContext, ParserErrorContext};
 use properties::{Importance, PropertyDeclaration, PropertyDeclarationBlock, PropertyId, PropertyParserContext};
 use properties::{PropertyDeclarationId, LonghandId, SourcePropertyDeclaration};
 use properties::LonghandIdSet;
@@ -218,11 +218,11 @@ impl Keyframe {
         let mut context = ParserContext::new(
             parent_stylesheet_contents.origin,
             &url_data,
-            &error_reporter,
             Some(CssRuleType::Keyframe),
             PARSING_MODE_DEFAULT,
             parent_stylesheet_contents.quirks_mode
         );
+        let error_context = ParserErrorContext { error_reporter: &error_reporter };
         context.namespaces = Some(&*namespaces);
         let mut input = ParserInput::new(css);
         let mut input = Parser::new(&mut input);
@@ -230,6 +230,7 @@ impl Keyframe {
         let mut declarations = SourcePropertyDeclaration::new();
         let mut rule_parser = KeyframeListParser {
             context: &context,
+            error_context: &error_context,
             shared_lock: &lock,
             declarations: &mut declarations,
         };
@@ -447,37 +448,42 @@ impl KeyframesAnimation {
 /// 40%, 60%, 100% {
 ///     width: 100%;
 /// }
-struct KeyframeListParser<'a> {
+struct KeyframeListParser<'a, R: 'a> {
     context: &'a ParserContext<'a>,
+    error_context: &'a ParserErrorContext<'a, R>,
     shared_lock: &'a SharedRwLock,
     declarations: &'a mut SourcePropertyDeclaration,
 }
 
 /// Parses a keyframe list from CSS input.
-pub fn parse_keyframe_list(
+pub fn parse_keyframe_list<R>(
     context: &ParserContext,
+    error_context: &ParserErrorContext<R>,
     input: &mut Parser,
     shared_lock: &SharedRwLock
-) -> Vec<Arc<Locked<Keyframe>>> {
+) -> Vec<Arc<Locked<Keyframe>>>
+    where R: ParseErrorReporter
+{
     debug_assert!(context.namespaces.is_some(),
                   "Parsing a keyframe list from a context without namespaces?");
 
     let mut declarations = SourcePropertyDeclaration::new();
     RuleListParser::new_for_nested_rule(input, KeyframeListParser {
         context: context,
+        error_context: error_context,
         shared_lock: shared_lock,
         declarations: &mut declarations,
     }).filter_map(Result::ok).collect()
 }
 
 enum Void {}
-impl<'a, 'i> AtRuleParser<'i> for KeyframeListParser<'a> {
+impl<'a, 'i, R> AtRuleParser<'i> for KeyframeListParser<'a, R> {
     type Prelude = Void;
     type AtRule = Arc<Locked<Keyframe>>;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
 }
 
-impl<'a, 'i> QualifiedRuleParser<'i> for KeyframeListParser<'a> {
+impl<'a, 'i, R: ParseErrorReporter> QualifiedRuleParser<'i> for KeyframeListParser<'a, R> {
     type Prelude = KeyframeSelector;
     type QualifiedRule = Arc<Locked<Keyframe>>;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -489,7 +495,7 @@ impl<'a, 'i> QualifiedRuleParser<'i> for KeyframeListParser<'a> {
             Ok(sel) => Ok(sel),
             Err(e) => {
                 let error = ContextualParseError::InvalidKeyframeRule(input.slice_from(start_position), e.clone());
-                self.context.log_css_error(start_location, error);
+                self.context.log_css_error(self.error_context, start_location, error);
                 Err(e)
             }
         }
@@ -518,7 +524,7 @@ impl<'a, 'i> QualifiedRuleParser<'i> for KeyframeListParser<'a> {
                 Err(err) => {
                     iter.parser.declarations.clear();
                     let error = ContextualParseError::UnsupportedKeyframePropertyDeclaration(err.slice, err.error);
-                    context.log_css_error(err.location, error);
+                    context.log_css_error(self.error_context, err.location, error);
                 }
             }
             // `parse_important` is not called here, `!important` is not allowed in keyframe blocks.

components/style/stylesheets/mod.rs

@@ -26,7 +26,7 @@ pub mod viewport_rule;
 
 use cssparser::{parse_one_rule, Parser, ParserInput};
 use error_reporting::NullReporter;
-use parser::ParserContext;
+use parser::{ParserContext, ParserErrorContext};
 use servo_arc::Arc;
 use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt;
@@ -230,7 +230,6 @@ impl CssRule {
         let context = ParserContext::new(
             parent_stylesheet_contents.origin,
             &url_data,
-            &error_reporter,
             None,
             PARSING_MODE_DEFAULT,
             parent_stylesheet_contents.quirks_mode,
@@ -246,6 +245,7 @@ impl CssRule {
         let mut rule_parser = TopLevelRuleParser {
             stylesheet_origin: parent_stylesheet_contents.origin,
             context: context,
+            error_context: ParserErrorContext { error_reporter: &error_reporter },
             shared_lock: &shared_lock,
             loader: loader,
             state: state,

components/style/stylesheets/rule_parser.rs

@@ -9,10 +9,10 @@ use computed_values::font_family::FamilyName;
 use counter_style::{parse_counter_style_body, parse_counter_style_name};
 use cssparser::{AtRuleParser, AtRuleType, Parser, QualifiedRuleParser, RuleListParser};
 use cssparser::{CowRcStr, SourceLocation, BasicParseError};
-use error_reporting::ContextualParseError;
+use error_reporting::{ContextualParseError, ParseErrorReporter};
 use font_face::parse_font_face_block;
 use media_queries::{parse_media_query_list, MediaList};
-use parser::{Parse, ParserContext};
+use parser::{Parse, ParserContext, ParserErrorContext};
 use properties::parse_property_declaration_list;
 use selector_parser::{SelectorImpl, SelectorParser};
 use selectors::SelectorList;
@@ -35,7 +35,7 @@ use values::KeyframesName;
 use values::specified::url::SpecifiedUrl;
 
 /// The parser for the top-level rules in a stylesheet.
-pub struct TopLevelRuleParser<'a> {
+pub struct TopLevelRuleParser<'a, R: 'a> {
     /// The origin of the stylesheet we're parsing.
     pub stylesheet_origin: Origin,
     /// A reference to the lock we need to use to create rules.
@@ -47,6 +47,8 @@ pub struct TopLevelRuleParser<'a> {
     /// This won't contain any namespaces, and only nested parsers created with
     /// `ParserContext::new_with_rule_type` will.
     pub context: ParserContext<'a>,
+    /// The context required for reporting parse errors.
+    pub error_context: ParserErrorContext<'a, R>,
     /// The current state of the parser.
     pub state: State,
     /// Whether we have tried to parse was invalid due to being in the wrong
@@ -59,12 +61,13 @@ pub struct TopLevelRuleParser<'a> {
     pub namespaces: &'a mut Namespaces,
 }
 
-impl<'b> TopLevelRuleParser<'b> {
-    fn nested<'a: 'b>(&'a self) -> NestedRuleParser<'a, 'b> {
+impl<'b, R> TopLevelRuleParser<'b, R> {
+    fn nested<'a: 'b>(&'a self) -> NestedRuleParser<'a, 'b, R> {
         NestedRuleParser {
             stylesheet_origin: self.stylesheet_origin,
             shared_lock: self.shared_lock,
             context: &self.context,
+            error_context: &self.error_context,
             namespaces: &self.namespaces,
         }
     }
@@ -147,7 +150,7 @@ fn register_namespace(_: &Namespace) -> Result<(), ()> {
     Ok(()) // servo doesn't use namespace ids
 }
 
-impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
+impl<'a, 'i, R: ParseErrorReporter> AtRuleParser<'i> for TopLevelRuleParser<'a, R> {
     type Prelude = AtRulePrelude;
     type AtRule = CssRule;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -256,7 +259,7 @@ pub struct QualifiedRuleParserPrelude {
     source_location: SourceLocation,
 }
 
-impl<'a, 'i> QualifiedRuleParser<'i> for TopLevelRuleParser<'a> {
+impl<'a, 'i, R: ParseErrorReporter> QualifiedRuleParser<'i> for TopLevelRuleParser<'a, R> {
     type Prelude = QualifiedRuleParserPrelude;
     type QualifiedRule = CssRule;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -281,14 +284,15 @@ impl<'a, 'i> QualifiedRuleParser<'i> for TopLevelRuleParser<'a> {
 }
 
 #[derive(Clone)]  // shallow, relatively cheap .clone
-struct NestedRuleParser<'a, 'b: 'a> {
+struct NestedRuleParser<'a, 'b: 'a, R: 'b> {
     stylesheet_origin: Origin,
     shared_lock: &'a SharedRwLock,
     context: &'a ParserContext<'b>,
+    error_context: &'a ParserErrorContext<'b, R>,
     namespaces: &'a Namespaces,
 }
 
-impl<'a, 'b> NestedRuleParser<'a, 'b> {
+impl<'a, 'b, R: ParseErrorReporter> NestedRuleParser<'a, 'b, R> {
     fn parse_nested_rules(
         &mut self,
         input: &mut Parser,
@@ -301,6 +305,7 @@ impl<'a, 'b> NestedRuleParser<'a, 'b> {
             stylesheet_origin: self.stylesheet_origin,
             shared_lock: self.shared_lock,
             context: &context,
+            error_context: &self.error_context,
             namespaces: self.namespaces,
         };
 
@@ -311,7 +316,7 @@ impl<'a, 'b> NestedRuleParser<'a, 'b> {
                 Ok(rule) => rules.push(rule),
                 Err(err) => {
                     let error = ContextualParseError::UnsupportedRule(err.slice, err.error);
-                    self.context.log_css_error(err.location, error);
+                    self.context.log_css_error(self.error_context, err.location, error);
                 }
             }
         }
@@ -319,7 +324,7 @@ impl<'a, 'b> NestedRuleParser<'a, 'b> {
     }
 }
 
-impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
+impl<'a, 'b, 'i, R: ParseErrorReporter> AtRuleParser<'i> for NestedRuleParser<'a, 'b, R> {
     type Prelude = AtRulePrelude;
     type AtRule = CssRule;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -428,7 +433,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                     );
 
                 Ok(CssRule::FontFace(Arc::new(self.shared_lock.wrap(
-                   parse_font_face_block(&context, input, location).into()))))
+                   parse_font_face_block(&context, self.error_context, input, location).into()))))
             }
             AtRulePrelude::FontFeatureValues(family_names, location) => {
                 let context =
@@ -438,7 +443,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                         self.namespaces,
                     );
                 Ok(CssRule::FontFeatureValues(Arc::new(self.shared_lock.wrap(
-                    FontFeatureValuesRule::parse(&context, input, family_names, location)))))
+                    FontFeatureValuesRule::parse(&context, self.error_context, input, family_names, location)))))
             }
             AtRulePrelude::CounterStyle(name) => {
                 let context =
@@ -448,7 +453,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                         self.namespaces,
                     );
                 Ok(CssRule::CounterStyle(Arc::new(self.shared_lock.wrap(
-                   parse_counter_style_body(name, &context, input)?.into()))))
+                   parse_counter_style_body(name, &context, self.error_context, input)?.into()))))
             }
             AtRulePrelude::Media(media_queries, location) => {
                 Ok(CssRule::Media(Arc::new(self.shared_lock.wrap(MediaRule {
@@ -480,7 +485,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                         self.namespaces,
                     );
                 Ok(CssRule::Viewport(Arc::new(self.shared_lock.wrap(
-                   ViewportRule::parse(&context, input)?))))
+                   ViewportRule::parse(&context, self.error_context, input)?))))
             }
             AtRulePrelude::Keyframes(name, prefix, location) => {
                 let context =
@@ -492,7 +497,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
 
                 Ok(CssRule::Keyframes(Arc::new(self.shared_lock.wrap(KeyframesRule {
                     name: name,
-                    keyframes: parse_keyframe_list(&context, input, self.shared_lock),
+                    keyframes: parse_keyframe_list(&context, self.error_context, input, self.shared_lock),
                     vendor_prefix: prefix,
                     source_location: location,
                 }))))
@@ -504,7 +509,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
                         CssRuleType::Page,
                         self.namespaces,
                     );
-                let declarations = parse_property_declaration_list(&context, input);
+                let declarations = parse_property_declaration_list(&context, self.error_context, input);
                 Ok(CssRule::Page(Arc::new(self.shared_lock.wrap(PageRule {
                     block: Arc::new(self.shared_lock.wrap(declarations)),
                     source_location: location,
@@ -525,7 +530,7 @@ impl<'a, 'b, 'i> AtRuleParser<'i> for NestedRuleParser<'a, 'b> {
     }
 }
 
-impl<'a, 'b, 'i> QualifiedRuleParser<'i> for NestedRuleParser<'a, 'b> {
+impl<'a, 'b, 'i, R: ParseErrorReporter> QualifiedRuleParser<'i> for NestedRuleParser<'a, 'b, R> {
     type Prelude = QualifiedRuleParserPrelude;
     type QualifiedRule = CssRule;
     type Error = SelectorParseError<'i, StyleParseError<'i>>;
@@ -561,7 +566,7 @@ impl<'a, 'b, 'i> QualifiedRuleParser<'i> for NestedRuleParser<'a, 'b> {
                 CssRuleType::Style,
                 self.namespaces,
             );
-        let declarations = parse_property_declaration_list(&context, input);
+        let declarations = parse_property_declaration_list(&context, self.error_context, input);
         Ok(CssRule::Style(Arc::new(self.shared_lock.wrap(StyleRule {
             selectors: prelude.selectors,
             block: Arc::new(self.shared_lock.wrap(declarations)),

components/style/stylesheets/stylesheet.rs

@@ -10,7 +10,7 @@ use fnv::FnvHashMap;
 use invalidation::media_queries::{MediaListKey, ToMediaListKey};
 use media_queries::{MediaList, Device};
 use parking_lot::RwLock;
-use parser::ParserContext;
+use parser::{ParserContext, ParserErrorContext};
 use servo_arc::Arc;
 use shared_lock::{DeepCloneParams, DeepCloneWithLock, Locked, SharedRwLock, SharedRwLockReadGuard};
 use std::mem;
@@ -67,13 +67,13 @@ pub struct StylesheetContents {
 impl StylesheetContents {
     /// Parse a given CSS string, with a given url-data, origin, and
     /// quirks mode.
-    pub fn from_str(
+    pub fn from_str<R: ParseErrorReporter>(
         css: &str,
         url_data: UrlExtraData,
         origin: Origin,
         shared_lock: &SharedRwLock,
         stylesheet_loader: Option<&StylesheetLoader>,
-        error_reporter: &ParseErrorReporter,
+        error_reporter: &R,
         quirks_mode: QuirksMode,
         line_number_offset: u64
     ) -> Self {
@@ -313,12 +313,14 @@ impl StylesheetInDocument for DocumentStyleSheet {
 
 impl Stylesheet {
     /// Updates an empty stylesheet from a given string of text.
-    pub fn update_from_str(existing: &Stylesheet,
-                           css: &str,
-                           url_data: UrlExtraData,
-                           stylesheet_loader: Option<&StylesheetLoader>,
-                           error_reporter: &ParseErrorReporter,
-                           line_number_offset: u64) {
+    pub fn update_from_str<R>(existing: &Stylesheet,
+                              css: &str,
+                              url_data: UrlExtraData,
+                              stylesheet_loader: Option<&StylesheetLoader>,
+                              error_reporter: &R,
+                              line_number_offset: u64)
+        where R: ParseErrorReporter
+    {
         let namespaces = RwLock::new(Namespaces::default());
         let (rules, dirty_on_viewport_size_change, source_map_url) =
             Stylesheet::parse_rules(
@@ -347,14 +349,14 @@ impl Stylesheet {
         *existing.contents.source_map_url.write() = source_map_url;
     }
 
-    fn parse_rules(
+    fn parse_rules<R: ParseErrorReporter>(
         css: &str,
         url_data: &UrlExtraData,
         origin: Origin,
         namespaces: &mut Namespaces,
         shared_lock: &SharedRwLock,
         stylesheet_loader: Option<&StylesheetLoader>,
-        error_reporter: &ParseErrorReporter,
+        error_reporter: &R,
         quirks_mode: QuirksMode,
         line_number_offset: u64
     ) -> (Vec<CssRule>, bool, Option<String>) {
@@ -366,17 +368,18 @@ impl Stylesheet {
             ParserContext::new_with_line_number_offset(
                 origin,
                 url_data,
-                error_reporter,
                 line_number_offset,
                 PARSING_MODE_DEFAULT,
                 quirks_mode
             );
+        let error_context = ParserErrorContext { error_reporter };
 
         let rule_parser = TopLevelRuleParser {
             stylesheet_origin: origin,
             shared_lock: shared_lock,
             loader: stylesheet_loader,
             context: context,
+            error_context: error_context,
             state: State::Start,
             had_hierarchy_error: false,
             namespaces: namespaces,
@@ -393,7 +396,8 @@ impl Stylesheet {
                     Ok(rule) => rules.push(rule),
                     Err(err) => {
                         let error = ContextualParseError::InvalidRule(err.slice, err.error);
-                        iter.parser.context.log_css_error(err.location, error);
+                        iter.parser.context.log_css_error(&iter.parser.error_context,
+                                                          err.location, error);
                     }
                 }
             }
@@ -408,16 +412,18 @@ impl Stylesheet {
     ///
     /// Effectively creates a new stylesheet and forwards the hard work to
     /// `Stylesheet::update_from_str`.
-    pub fn from_str(css: &str,
-                    url_data: UrlExtraData,
-                    origin: Origin,
-                    media: Arc<Locked<MediaList>>,
-                    shared_lock: SharedRwLock,
-                    stylesheet_loader: Option<&StylesheetLoader>,
-                    error_reporter: &ParseErrorReporter,
-                    quirks_mode: QuirksMode,
-                    line_number_offset: u64)
-                    -> Stylesheet {
+    pub fn from_str<R: ParseErrorReporter>(
+        css: &str,
+        url_data: UrlExtraData,
+        origin: Origin,
+        media: Arc<Locked<MediaList>>,
+        shared_lock: SharedRwLock,
+        stylesheet_loader: Option<&StylesheetLoader>,
+        error_reporter: &R,
+        quirks_mode: QuirksMode,
+        line_number_offset: u64)
+        -> Stylesheet
+    {
         let contents = StylesheetContents::from_str(
             css,
             url_data,

components/style/stylesheets/viewport_rule.rs

@@ -11,11 +11,11 @@ use app_units::Au;
 use context::QuirksMode;
 use cssparser::{AtRuleParser, DeclarationListParser, DeclarationParser, Parser, parse_important};
 use cssparser::{CowRcStr, ToCss as ParserToCss};
-use error_reporting::ContextualParseError;
+use error_reporting::{ContextualParseError, ParseErrorReporter};
 use euclid::TypedSize2D;
 use font_metrics::get_metrics_provider_for_product;
 use media_queries::Device;
-use parser::{Parse, ParserContext};
+use parser::{ParserContext, ParserErrorContext};
 use properties::StyleBuilder;
 use selectors::parser::SelectorParseError;
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
@@ -346,8 +346,14 @@ fn is_whitespace_separator_or_equals(c: &char) -> bool {
     WHITESPACE.contains(c) || SEPARATOR.contains(c) || *c == '='
 }
 
-impl Parse for ViewportRule {
-    fn parse<'i, 't>(context: &ParserContext, input: &mut Parser<'i, 't>) -> Result<Self, ParseError<'i>> {
+impl ViewportRule {
+    /// Parse a single @viewport rule.
+    pub fn parse<'i, 't, R>(context: &ParserContext,
+                            error_context: &ParserErrorContext<R>,
+                            input: &mut Parser<'i, 't>)
+                            -> Result<Self, ParseError<'i>>
+        where R: ParseErrorReporter
+    {
         let parser = ViewportRuleParser { context: context };
 
         let mut cascade = Cascade::new();
@@ -361,7 +367,7 @@ impl Parse for ViewportRule {
                 }
                 Err(err) => {
                     let error = ContextualParseError::UnsupportedViewportDescriptorDeclaration(err.slice, err.error);
-                    context.log_css_error(err.location, error);
+                    context.log_css_error(error_context, err.location, error);
                 }
             }
         }