style: Make :host() and ::slotted() account for the inner selector's specificity.

As resolved in https://github.com/w3c/csswg-drafts/issues/1915.

Bug: 1454165
Reviewed-by: xidorn
Differential Revision: https://phabricator.services.mozilla.com/D1849

components/selectors/builder.rs

@@ -23,7 +23,7 @@ use sink::Push;
 use smallvec::{self, SmallVec};
 use std::cmp;
 use std::iter;
-use std::ops::Add;
+use std::ops::{AddAssign, Add};
 use std::ptr;
 use std::slice;
 
@@ -228,6 +228,16 @@ struct Specificity {
     element_selectors: u32,
 }
 
+
+impl AddAssign for Specificity {
+    #[inline]
+    fn add_assign(&mut self, rhs: Self) {
+        self.id_selectors += rhs.id_selectors;
+        self.class_like_selectors += rhs.class_like_selectors;
+        self.element_selectors += rhs.element_selectors;
+    }
+}
+
 impl Add for Specificity {
     type Output = Specificity;
 
@@ -251,6 +261,7 @@ impl Default for Specificity {
 }
 
 impl From<u32> for Specificity {
+    #[inline]
     fn from(value: u32) -> Specificity {
         assert!(value <= MAX_10BIT << 20 | MAX_10BIT << 10 | MAX_10BIT);
         Specificity {
@@ -262,6 +273,7 @@ impl From<u32> for Specificity {
 }
 
 impl From<Specificity> for u32 {
+    #[inline]
     fn from(specificity: Specificity) -> u32 {
         cmp::min(specificity.id_selectors, MAX_10BIT) << 20 |
             cmp::min(specificity.class_like_selectors, MAX_10BIT) << 10 |
@@ -298,17 +310,29 @@ where
                     builder,
                 );
             }
-            // FIXME(emilio): Spec doesn't define any particular specificity for
+            Component::PseudoElement(..) | Component::LocalName(..) => {
-            // ::slotted(), so apply the general rule for pseudos per:
-            //
-            // https://github.com/w3c/csswg-drafts/issues/1915
-            //
-            // Though other engines compute it dynamically, so maybe we should
-            // do that instead, eventually.
-            Component::Slotted(..) | Component::PseudoElement(..) | Component::LocalName(..) => {
                 specificity.element_selectors += 1
             },
-            Component::ID(..) => specificity.id_selectors += 1,
+            Component::Slotted(ref selector) => {
+                specificity.element_selectors += 1;
+                // Note that due to the way ::slotted works we only compete with
+                // other ::slotted rules, so the above rule doesn't really
+                // matter, but we do it still for consistency with other
+                // pseudo-elements.
+                //
+                // See: https://github.com/w3c/csswg-drafts/issues/1915
+                *specificity += Specificity::from(selector.specificity());
+            },
+            Component::Host(ref selector) => {
+                specificity.class_like_selectors += 1;
+                if let Some(ref selector) = *selector {
+                    // See: https://github.com/w3c/csswg-drafts/issues/1915
+                    *specificity += Specificity::from(selector.specificity());
+                }
+            }
+            Component::ID(..) => {
+                specificity.id_selectors += 1;
+            },
             Component::Class(..) |
             Component::AttributeInNoNamespace { .. } |
             Component::AttributeInNoNamespaceExists { .. } |
@@ -319,7 +343,6 @@ where
             Component::Root |
             Component::Empty |
             Component::Scope |
-            Component::Host(..) |
             Component::NthChild(..) |
             Component::NthLastChild(..) |
             Component::NthOfType(..) |
@@ -327,7 +350,9 @@ where
             Component::FirstOfType |
             Component::LastOfType |
             Component::OnlyOfType |
-            Component::NonTSPseudoClass(..) => specificity.class_like_selectors += 1,
+            Component::NonTSPseudoClass(..) => {
+                specificity.class_like_selectors += 1;
+            },
             Component::ExplicitUniversalType |
             Component::ExplicitAnyNamespace |
             Component::ExplicitNoNamespace |