style: Implement @import layer|layer(<name>)

This works modulo the existing nested layer order bug. Will be covered
by WPT /css/css-cascade/layer-import.html once the feature is enabled (I
can probably enable it right away for those tests, but I'd rather fix
the obvious bugs first).

Differential Revision: https://phabricator.services.mozilla.com/D124538

4 files changed, 98 insertions, 25 deletions

diff --git a/components/style/stylesheets/import_rule.rs b/components/style/stylesheets/import_rule.rs
index 8f518f29bac..d60802cf0ab 100644
--- a/components/style/stylesheets/import_rule.rs
+++ b/components/style/stylesheets/import_rule.rs
@@ -11,6 +11,7 @@ use crate::shared_lock::{DeepCloneParams, DeepCloneWithLock};
 use crate::shared_lock::{SharedRwLock, SharedRwLockReadGuard, ToCssWithGuard};
 use crate::str::CssStringWriter;
 use crate::stylesheets::{CssRule, StylesheetInDocument};
+use crate::stylesheets::layer_rule::LayerName;
 use crate::values::CssUrl;
 use cssparser::SourceLocation;
 use std::fmt::{self, Write};
@@ -135,6 +136,31 @@ impl DeepCloneWithLock for ImportSheet {
     }
 }
 
+/// The layer keyword or function in an import rule.
+#[derive(Debug)]
+pub struct ImportLayer {
+    /// Whether the layer is anonymous.
+    pub is_anonymous: bool,
+    /// The layer name.
+    pub name: LayerName,
+}
+
+
+impl ToCss for ImportLayer {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        if self.is_anonymous {
+            dest.write_str("layer")
+        } else {
+            dest.write_str("layer(")?;
+            self.name.to_css(dest)?;
+            dest.write_char(')')
+        }
+    }
+}
+
 /// The [`@import`][import] at-rule.
 ///
 /// [import]: https://drafts.csswg.org/css-cascade-3/#at-import
@@ -148,6 +174,9 @@ pub struct ImportRule {
     /// ImportSheet just has stub behavior until it appears.
     pub stylesheet: ImportSheet,
 
+    /// A `layer()` function name.
+    pub layer: Option<ImportLayer>,
+
     /// The line and column of the rule's source code.
     pub source_location: SourceLocation,
 }
@@ -170,6 +199,16 @@ impl DeepCloneWithLock for ImportRule {
         ImportRule {
             url: self.url.clone(),
             stylesheet: self.stylesheet.deep_clone_with_lock(lock, guard, params),
+            layer: self.layer.as_ref().map(|layer| {
+                ImportLayer {
+                    is_anonymous: layer.is_anonymous,
+                    name: if layer.is_anonymous {
+                        LayerName::new_anonymous()
+                    } else {
+                        layer.name.clone()
+                    },
+                }
+            }),
             source_location: self.source_location.clone(),
         }
     }
@@ -180,14 +219,18 @@ impl ToCssWithGuard for ImportRule {
         dest.write_str("@import ")?;
         self.url.to_css(&mut CssWriter::new(dest))?;
 
-        match self.stylesheet.media(guard) {
-            Some(media) if !media.is_empty() => {
-                dest.write_str(" ")?;
+        if let Some(media) = self.stylesheet.media(guard) {
+            if !media.is_empty() {
+                dest.write_char(' ')?;
                 media.to_css(&mut CssWriter::new(dest))?;
-            },
-            _ => {},
-        };
+            }
+        }
+
+        if let Some(ref layer) = self.layer {
+            dest.write_char(' ')?;
+            layer.to_css(&mut CssWriter::new(dest))?;
+        }
 
-        dest.write_str(";")
+        dest.write_char(';')
     }
 }
diff --git a/components/style/stylesheets/loader.rs b/components/style/stylesheets/loader.rs
index ce7c563db05..c145bafdc04 100644
--- a/components/style/stylesheets/loader.rs
+++ b/components/style/stylesheets/loader.rs
@@ -8,7 +8,7 @@
 use crate::media_queries::MediaList;
 use crate::parser::ParserContext;
 use crate::shared_lock::{Locked, SharedRwLock};
-use crate::stylesheets::import_rule::ImportRule;
+use crate::stylesheets::import_rule::{ImportRule, ImportLayer};
 use crate::values::CssUrl;
 use cssparser::SourceLocation;
 use servo_arc::Arc;
@@ -25,5 +25,6 @@ pub trait StylesheetLoader {
         context: &ParserContext,
         lock: &SharedRwLock,
         media: Arc<Locked<MediaList>>,
+        layer: Option<ImportLayer>,
     ) -> Arc<Locked<ImportRule>>;
 }
diff --git a/components/style/stylesheets/rule_parser.rs b/components/style/stylesheets/rule_parser.rs
index 7fb76a8d7a9..421431a8c10 100644
--- a/components/style/stylesheets/rule_parser.rs
+++ b/components/style/stylesheets/rule_parser.rs
@@ -18,6 +18,7 @@ use crate::stylesheets::font_feature_values_rule::parse_family_name_list;
 use crate::stylesheets::keyframes_rule::parse_keyframe_list;
 use crate::stylesheets::stylesheet::Namespaces;
 use crate::stylesheets::supports_rule::SupportsCondition;
+use crate::stylesheets::import_rule::ImportLayer;
 use crate::stylesheets::layer_rule::{LayerName, LayerRuleKind};
 use crate::stylesheets::viewport_rule;
 use crate::stylesheets::AllowImportRules;
@@ -169,7 +170,7 @@ pub enum AtRulePrelude {
     /// A @document rule, with its conditional.
     Document(DocumentCondition),
     /// A @import rule prelude.
-    Import(CssUrl, Arc<Locked<MediaList>>),
+    Import(CssUrl, Arc<Locked<MediaList>>, Option<ImportLayer>),
     /// A @namespace rule prelude.
     Namespace(Option<Prefix>, Namespace),
     /// A @layer rule prelude.
@@ -206,10 +207,29 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                 let url_string = input.expect_url_or_string()?.as_ref().to_owned();
                 let url = CssUrl::parse_from_string(url_string, &self.context, CorsMode::None);
 
+                let layer = if !static_prefs::pref!("layout.css.cascade-layers.enabled") {
+                    None
+                } else if input.try_parse(|input| input.expect_ident_matching("layer")).is_ok() {
+                    Some(ImportLayer {
+                        is_anonymous: true,
+                        name: LayerName::new_anonymous(),
+                    })
+                } else {
+                    input.try_parse(|input| {
+                        input.expect_function_matching("layer")?;
+                        input.parse_nested_block(|input| {
+                            LayerName::parse(&self.context, input)
+                        }).map(|name| ImportLayer {
+                            is_anonymous: false,
+                            name,
+                        })
+                    }).ok()
+                };
+
                 let media = MediaList::parse(&self.context, input);
                 let media = Arc::new(self.shared_lock.wrap(media));
 
-                return Ok(AtRulePrelude::Import(url, media));
+                return Ok(AtRulePrelude::Import(url, media, layer));
             },
             "namespace" => {
                 if !self.check_state(State::Namespaces) {
@@ -263,7 +283,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
         start: &ParserState,
     ) -> Result<Self::AtRule, ()>  {
         let rule = match prelude {
-            AtRulePrelude::Import(url, media) => {
+            AtRulePrelude::Import(url, media, layer) => {
                 let loader = self
                     .loader
                     .expect("Expected a stylesheet loader for @import");
@@ -274,6 +294,7 @@ impl<'a, 'i> AtRuleParser<'i> for TopLevelRuleParser<'a> {
                     &self.context,
                     &self.shared_lock,
                     media,
+                    layer,
                 );
 
                 self.state = State::Imports;
diff --git a/components/style/stylist.rs b/components/style/stylist.rs
index 0a87da95bc8..1c90886e17d 100644
--- a/components/style/stylist.rs
+++ b/components/style/stylist.rs
@@ -2329,15 +2329,36 @@ impl CascadeData {
                 continue;
             }
 
+
+            fn maybe_register_layer(data: &mut CascadeData, layer: &LayerName) -> u32 {
+                // TODO: Measure what's more common / expensive, if
+                // layer.clone() or the double hash lookup in the insert
+                // case.
+                if let Some(order) = data.layer_order.get(layer) {
+                    return *order;
+                }
+                let order = data.next_layer_order;
+                data.layer_order.insert(layer.clone(), order);
+                data.next_layer_order += 1;
+                order
+            }
+
             let mut layer_names_to_pop = 0;
             let mut children_layer_order = current_layer_order;
             match *rule {
                 CssRule::Import(ref lock) => {
+                    let import_rule = lock.read_with(guard);
                     if rebuild_kind.should_rebuild_invalidation() {
-                        let import_rule = lock.read_with(guard);
                         self.effective_media_query_results
                             .saw_effective(import_rule);
                     }
+                    if let Some(ref layer) = import_rule.layer {
+                        for name in layer.name.layer_names() {
+                            current_layer.0.push(name.clone());
+                            children_layer_order = maybe_register_layer(self, &current_layer);
+                            layer_names_to_pop += 1;
+                        }
+                    }
 
                 },
                 CssRule::Media(ref lock) => {
@@ -2349,19 +2370,6 @@ impl CascadeData {
                 CssRule::Layer(ref lock) => {
                     use crate::stylesheets::layer_rule::LayerRuleKind;
 
-                    fn maybe_register_layer(data: &mut CascadeData, layer: &LayerName) -> u32 {
-                        // TODO: Measure what's more common / expensive, if
-                        // layer.clone() or the double hash lookup in the insert
-                        // case.
-                        if let Some(order) = data.layer_order.get(layer) {
-                            return *order;
-                        }
-                        let order = data.next_layer_order;
-                        data.layer_order.insert(layer.clone(), order);
-                        data.next_layer_order += 1;
-                        order
-                    }
-
                     let layer_rule = lock.read_with(guard);
                     match layer_rule.kind {
                         LayerRuleKind::Block { ref name, .. } => {