style: Implement shadow part forwarding (minus invalidation).

Some of the stuff, in particular inside GeckoBindings stuff should be
refactored to be less ugly and duplicate a bit less code, but the rest of the
code should be landable as is.

Some invalidation changes are already needed because we weren't matching with
the right shadow host during invalidation (which made existing ::part() tests
fail).

Pending invalidation work:

 * Making exportparts work right on the snapshots.
 * Invalidating parts from descendant hosts.

They're not very hard but I need to think how to best implement it:

 * Maybe get rid of ShadowRoot::mParts and just walk DOM descendants in the
   Shadow DOM.

 * Maybe implement a ElementHasExportPartsAttr much like HasPartAttr and use
   that to keep the list of elements.

 * Maybe invalidate :host and ::part() together in here[1]

 * Maybe something else.

Opinions?

[1]: https://searchfox.org/mozilla-central/rev/131338e5017bc0283d86fb73844407b9a2155c98/servo/components/style/invalidation/element/invalidator.rs#561

Differential Revision: https://phabricator.services.mozilla.com/D53730

components/selectors/matching.rs

@@ -8,6 +8,7 @@ use crate::nth_index_cache::NthIndexCacheInner;
 use crate::parser::{AncestorHashes, Combinator, Component, LocalName};
 use crate::parser::{NonTSPseudoClass, Selector, SelectorImpl, SelectorIter, SelectorList};
 use crate::tree::Element;
+use smallvec::SmallVec;
 use std::borrow::Borrow;
 use std::iter;
 
@@ -667,7 +668,41 @@ where
 
     match *selector {
         Component::Combinator(_) => unreachable!(),
-        Component::Part(ref parts) => parts.iter().all(|part| element.is_part(part)),
+        Component::Part(ref parts) => {
+            let mut hosts = SmallVec::<[E; 4]>::new();
+
+            let mut host = match element.containing_shadow_host() {
+                Some(h) => h,
+                None => return false,
+            };
+
+            loop {
+                let outer_host = host.containing_shadow_host();
+                if outer_host.as_ref().map(|h| h.opaque()) == context.shared.current_host {
+                    break;
+                }
+                let outer_host = match outer_host {
+                    Some(h) => h,
+                    None => return false,
+                };
+                // TODO(emilio): if worth it, we could early return if
+                // host doesn't have the exportparts attribute.
+                hosts.push(host);
+                host = outer_host;
+            }
+
+            // Translate the part into the right scope.
+            parts.iter().all(|part| {
+                let mut part = part.clone();
+                for host in hosts.iter().rev() {
+                    part = match host.imported_part(&part) {
+                        Some(p) => p,
+                        None => return false,
+                    };
+                }
+                element.is_part(&part)
+            })
+        },
         Component::Slotted(ref selector) => {
             // <slots> are never flattened tree slottables.
             !element.is_html_slot_element() &&

components/selectors/tree.rs

@@ -117,6 +117,20 @@ pub trait Element: Sized + Clone + Debug {
         case_sensitivity: CaseSensitivity,
     ) -> bool;
 
+    /// Returns the mapping from the `exportparts` attribute in the regular
+    /// direction, that is, inner-tree -> outer-tree.
+    fn exported_part(
+        &self,
+        name: &<Self::Impl as SelectorImpl>::PartName,
+    ) -> Option<<Self::Impl as SelectorImpl>::PartName>;
+
+    /// Returns the mapping from the `exportparts` attribute in the reverse
+    /// direction, that is, in an outer-tree -> inner-tree direction.
+    fn imported_part(
+        &self,
+        name: &<Self::Impl as SelectorImpl>::PartName,
+    ) -> Option<<Self::Impl as SelectorImpl>::PartName>;
+
     fn is_part(&self, name: &<Self::Impl as SelectorImpl>::PartName) -> bool;
 
     /// Returns whether this element matches `:empty`.

components/style/dom_apis.rs

@@ -172,7 +172,11 @@ where
         };
 
         for selector in self.selector_list.0.iter() {
-            target_vector.push(Invalidation::new(selector, 0))
+            target_vector.push(Invalidation::new(
+                selector,
+                self.matching_context.current_host.clone(),
+                0,
+            ))
         }
 
         false

components/style/gecko/wrapper.rs

@@ -2216,6 +2216,28 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         snapshot_helpers::has_class_or_part(name, CaseSensitivity::CaseSensitive, attr)
     }
 
+    #[inline]
+    fn imported_part(&self, name: &Atom) -> Option<Atom> {
+        let imported = unsafe {
+            bindings::Gecko_Element_ImportedPart(self.0, name.as_ptr())
+        };
+        if imported.is_null() {
+            return None;
+        }
+        Some(unsafe { Atom::from_raw(imported) })
+    }
+
+    #[inline]
+    fn exported_part(&self, name: &Atom) -> Option<Atom> {
+        let exported = unsafe {
+            bindings::Gecko_Element_ExportedPart(self.0, name.as_ptr())
+        };
+        if exported.is_null() {
+            return None;
+        }
+        Some(unsafe { Atom::from_raw(exported) })
+    }
+
     #[inline(always)]
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         let attr = match self.get_class_attr() {

components/style/invalidation/element/document_state.rs

@@ -79,7 +79,13 @@ where
                     continue;
                 }
 
-                self_invalidations.push(Invalidation::new(&dependency.selector, 0));
+                // We pass `None` as a scope, as document state selectors aren't
+                // affected by the current scope.
+                self_invalidations.push(Invalidation::new(
+                    &dependency.selector,
+                    /* scope = */ None,
+                    0,
+                ));
             }
         }
 

components/style/invalidation/element/element_wrapper.rs

@@ -365,6 +365,16 @@ where
         }
     }
 
+    fn exported_part(&self, name: &Atom) -> Option<Atom> {
+        // FIXME(emilio): Implement for proper invalidation.
+        self.element.exported_part(name)
+    }
+
+    fn imported_part(&self, name: &Atom) -> Option<Atom> {
+        // FIXME(emilio): Implement for proper invalidation.
+        self.element.imported_part(name)
+    }
+
     fn has_class(&self, name: &Atom, case_sensitivity: CaseSensitivity) -> bool {
         match self.snapshot() {
             Some(snapshot) if snapshot.has_attrs() => snapshot.has_class(name, case_sensitivity),

components/style/invalidation/element/invalidator.rs

@@ -8,6 +8,7 @@
 use crate::context::StackLimitChecker;
 use crate::dom::{TElement, TNode, TShadowRoot};
 use crate::selector_parser::SelectorImpl;
+use selectors::OpaqueElement;
 use selectors::matching::matches_compound_selector_from;
 use selectors::matching::{CompoundSelectorMatchingResult, MatchingContext};
 use selectors::parser::{Combinator, Component, Selector};
@@ -127,6 +128,8 @@ enum InvalidationKind {
 #[derive(Clone)]
 pub struct Invalidation<'a> {
     selector: &'a Selector<SelectorImpl>,
+    /// The right shadow host from where the rule came from, if any.
+    scope: Option<OpaqueElement>,
     /// The offset of the selector pointing to a compound selector.
     ///
     /// This order is a "parse order" offset, that is, zero is the leftmost part
@@ -143,9 +146,14 @@ pub struct Invalidation<'a> {
 
 impl<'a> Invalidation<'a> {
     /// Create a new invalidation for a given selector and offset.
-    pub fn new(selector: &'a Selector<SelectorImpl>, offset: usize) -> Self {
+    pub fn new(
+        selector: &'a Selector<SelectorImpl>,
+        scope: Option<OpaqueElement>,
+        offset: usize,
+    ) -> Self {
         Self {
             selector,
+            scope,
             offset,
             matched_by_any_previous: false,
         }
@@ -483,6 +491,9 @@ where
 
         let mut any = false;
         let mut sibling_invalidations = InvalidationVector::new();
+
+        // FIXME(emilio): We also need to invalidate parts in descendant shadow
+        // hosts that have exportparts attributes.
         for element in shadow.parts() {
             any |= self.invalidate_child(
                 *element,
@@ -721,12 +732,17 @@ where
             self.element, invalidation, invalidation_kind
         );
 
-        let matching_result = matches_compound_selector_from(
-            &invalidation.selector,
-            invalidation.offset,
-            self.processor.matching_context(),
-            &self.element,
-        );
+        let matching_result = {
+            let mut context = self.processor.matching_context();
+            context.current_host = invalidation.scope;
+
+            matches_compound_selector_from(
+                &invalidation.selector,
+                invalidation.offset,
+                context,
+                &self.element,
+            )
+        };
 
         let mut invalidated_self = false;
         let mut matched = false;
@@ -809,6 +825,7 @@ where
 
                 let next_invalidation = Invalidation {
                     selector: invalidation.selector,
+                    scope: invalidation.scope,
                     offset: next_combinator_offset + 1,
                     matched_by_any_previous: false,
                 };

components/style/invalidation/element/state_and_attributes.rs

@@ -457,6 +457,7 @@ where
 
         let invalidation = Invalidation::new(
             &dependency.selector,
+            self.matching_context.current_host.clone(),
             dependency.selector.len() - dependency.selector_offset + 1,
         );
 

components/style/rule_collector.rs

@@ -4,6 +4,7 @@
 
 //! Collects a series of applicable rules for a given element.
 
+use crate::Atom;
 use crate::applicable_declarations::{ApplicableDeclarationBlock, ApplicableDeclarationList};
 use crate::dom::{TElement, TNode, TShadowRoot};
 use crate::properties::{AnimationRules, PropertyDeclarationBlock};
@@ -328,52 +329,75 @@ where
             return;
         }
 
-        let shadow = match self.rule_hash_target.containing_shadow() {
+        let mut inner_shadow = match self.rule_hash_target.containing_shadow() {
             Some(s) => s,
             None => return,
         };
 
-        let host = shadow.host();
-        let containing_shadow = host.containing_shadow();
-        let part_rules = match containing_shadow {
-            Some(shadow) => shadow
-                .style_data()
-                .and_then(|data| data.part_rules(self.pseudo_element)),
-            None => self
-                .stylist
-                .cascade_data()
-                .borrow_for_origin(Origin::Author)
-                .part_rules(self.pseudo_element),
-        };
+        let mut shadow_cascade_order = ShadowCascadeOrder::for_innermost_containing_tree();
 
-        // TODO(emilio): Cascade order will need to increment for each tree when
-        // we implement forwarding.
-        let shadow_cascade_order = ShadowCascadeOrder::for_innermost_containing_tree();
-        if let Some(part_rules) = part_rules {
-            let containing_host = containing_shadow.map(|s| s.host());
-            let element = self.element;
-            let rule_hash_target = self.rule_hash_target;
-            let rules = &mut self.rules;
-            let flags_setter = &mut self.flags_setter;
-            let cascade_level = CascadeLevel::AuthorNormal {
-                shadow_cascade_order,
+        let mut parts = SmallVec::<[Atom; 3]>::new();
+        self.rule_hash_target.each_part(|p| parts.push(p.clone()));
+
+        loop {
+            if parts.is_empty() {
+                return;
+            }
+
+            let outer_shadow = inner_shadow.host().containing_shadow();
+            let part_rules = match outer_shadow {
+                Some(shadow) => shadow
+                    .style_data()
+                    .and_then(|data| data.part_rules(self.pseudo_element)),
+                None => self
+                    .stylist
+                    .cascade_data()
+                    .borrow_for_origin(Origin::Author)
+                    .part_rules(self.pseudo_element),
             };
-            let start = rules.len();
-            self.context.with_shadow_host(containing_host, |context| {
-                rule_hash_target.each_part(|p| {
-                    if let Some(part_rules) = part_rules.get(p) {
-                        SelectorMap::get_matching_rules(
-                            element,
-                            &part_rules,
-                            rules,
-                            context,
-                            flags_setter,
-                            cascade_level,
-                        );
+
+            if let Some(part_rules) = part_rules {
+                let containing_host = outer_shadow.map(|s| s.host());
+                let element = self.element;
+                let rules = &mut self.rules;
+                let flags_setter = &mut self.flags_setter;
+                let cascade_level = CascadeLevel::AuthorNormal {
+                    shadow_cascade_order,
+                };
+                let start = rules.len();
+                self.context.with_shadow_host(containing_host, |context| {
+                    for p in &parts {
+                        if let Some(part_rules) = part_rules.get(p) {
+                            SelectorMap::get_matching_rules(
+                                element,
+                                &part_rules,
+                                rules,
+                                context,
+                                flags_setter,
+                                cascade_level,
+                            );
+                        }
                     }
                 });
+                sort_rules_from(rules, start);
+                shadow_cascade_order.inc();
+            }
+
+            let inner_shadow_host = inner_shadow.host();
+
+            inner_shadow = match outer_shadow {
+                Some(s) => s,
+                None => break, // Nowhere to export to.
+            };
+
+            parts.retain(|part| {
+                let exported_part = match inner_shadow_host.exported_part(part) {
+                    Some(part) => part,
+                    None => return false,
+                };
+                std::mem::replace(part, exported_part);
+                true
             });
-            sort_rules_from(rules, start);
         }
     }