Auto merge of #19842 - emilio:cascade-cleanup, r=nox,jryans

style: Cleanup a bit the cascade.

This makes an element available in StyleAdjuster, and uses it to replace some of our CascadeFlags (the ones that don't represent restrictions on what's actually cascaded, actually).

That fixes handwaviness in the handling of those flags from style reparenting,
and code duplication to handle tricky stuff like :visited.

There are a number of other changes that are worth noticing:

 * skip_root_and_item_based_display_fixup is renamed to skip_item_display_fixup:

   TElement::is_root() already implies being the document element, which by
   definition is not native anonymous and not a pseudo-element.

   Thus, you never get fixed-up if your NAC or a pseudo, which is what the code
   tried to avoid, so the only fixup with a point is the item one, which is
   necessary.

 * The pseudo-element probing code was refactored to return early a
   Option::<CascadeInputs>::None, which is nicer than what it was doing.

 * The visited_links_enabled check has moved to selector-matching time. The rest
   of the checks aren't based on whether the element is a link, or are properly
   guarded by parent_style.visited_style().is_some() or visited_rules.is_some().

   Thus you can transitively infer that no element will end up with a :visited
   style, not even from style reparenting.

Anyway, the underlying reason why I want the element in StyleAdjuster is because
we're going to implement an adjustment in there depending on the tag of the
element (converting display: contents to display: none depending on the tag), so
computing that information eagerly, including a hash lookup, wouldn't be nice.

See each commit for details.

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2 files changed, 13 insertions, 6 deletions

diff --git a/components/script_layout_interface/lib.rs b/components/script_layout_interface/lib.rs
index 0538aabafd0..45c82ee1089 100644
--- a/components/script_layout_interface/lib.rs
+++ b/components/script_layout_interface/lib.rs
@@ -7,6 +7,7 @@
 //! to depend on script.
 
 #![deny(unsafe_code)]
+#![feature(associated_type_defaults)]
 
 extern crate app_units;
 extern crate atomic_refcell;
diff --git a/components/script_layout_interface/wrapper_traits.rs b/components/script_layout_interface/wrapper_traits.rs
index dc1f5db4c54..0b04d4256a6 100644
--- a/components/script_layout_interface/wrapper_traits.rs
+++ b/components/script_layout_interface/wrapper_traits.rs
@@ -19,10 +19,10 @@ use std::fmt::Debug;
 use style::attr::AttrValue;
 use style::context::SharedStyleContext;
 use style::data::ElementData;
-use style::dom::{LayoutIterator, NodeInfo, TNode};
+use style::dom::{LayoutIterator, NodeInfo, TElement, TNode};
 use style::dom::OpaqueNode;
 use style::font_metrics::ServoMetricsProvider;
-use style::properties::{CascadeFlags, ComputedValues};
+use style::properties::ComputedValues;
 use style::selector_parser::{PseudoElement, PseudoElementCascadeType, SelectorImpl};
 use style::stylist::RuleInclusion;
 use webrender_api::ClipId;
@@ -148,6 +148,8 @@ impl<ConcreteNode> Iterator for TreeIterator<ConcreteNode>
 /// node does not allow any parents or siblings of nodes to be accessed, to avoid races.
 pub trait ThreadSafeLayoutNode: Clone + Copy + Debug + GetLayoutData + NodeInfo + PartialEq + Sized {
     type ConcreteNode: LayoutNode<ConcreteThreadSafeLayoutNode = Self>;
+    type ConcreteElement: TElement;
+
     type ConcreteThreadSafeLayoutElement:
         ThreadSafeLayoutElement<ConcreteThreadSafeLayoutNode = Self>
         + ::selectors::Element<Impl=SelectorImpl>;
@@ -291,6 +293,12 @@ pub trait ThreadSafeLayoutElement
 {
     type ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<ConcreteThreadSafeLayoutElement = Self>;
 
+    /// This type alias is just a work-around to avoid writing
+    ///
+    ///   <Self::ConcreteThreadSafeLayoutNode as ThreadSafeLayoutNode>::ConcreteElement
+    ///
+    type ConcreteElement: TElement;
+
     fn as_node(&self) -> Self::ConcreteThreadSafeLayoutNode;
 
     /// Creates a new `ThreadSafeLayoutElement` for the same `LayoutElement`
@@ -307,8 +315,7 @@ pub trait ThreadSafeLayoutElement
     ///
     /// We need this so that the functions defined on this trait can call
     /// lazily_compute_pseudo_element_style, which operates on TElement.
-    unsafe fn unsafe_get(self) ->
-        <<Self::ConcreteThreadSafeLayoutNode as ThreadSafeLayoutNode>::ConcreteNode as TNode>::ConcreteElement;
+    unsafe fn unsafe_get(self) -> Self::ConcreteElement;
 
     #[inline]
     fn get_attr(&self, namespace: &Namespace, name: &LocalName) -> Option<&str>;
@@ -382,11 +389,10 @@ pub trait ThreadSafeLayoutElement
                             .unwrap().clone()
                     },
                     PseudoElementCascadeType::Precomputed => {
-                        context.stylist.precomputed_values_for_pseudo(
+                        context.stylist.precomputed_values_for_pseudo::<Self::ConcreteElement>(
                             &context.guards,
                             &style_pseudo,
                             Some(data.styles.primary()),
-                            CascadeFlags::empty(),
                             &ServoMetricsProvider,
                         )
                     }