Auto merge of #16874 - bholley:applicable_declarations_smallvec, r=emilio

Use a SmallVec when gathering applicable declarations

https://bugzilla.mozilla.org/show_bug.cgi?id=1364952

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components/style/matching.rs

@@ -29,7 +29,7 @@ use selectors::matching::AFFECTED_BY_PSEUDO_ELEMENTS;
 use shared_lock::StylesheetGuards;
 use sink::ForgetfulSink;
 use stylearc::Arc;
-use stylist::ApplicableDeclarationBlock;
+use stylist::ApplicableDeclarationList;
 
 /// The way a style should be inherited.
 enum InheritMode {
@@ -865,11 +865,11 @@ trait PrivateMatchMethods: TElement {
 }
 
 fn compute_rule_node<E: TElement>(rule_tree: &RuleTree,
-                                  applicable_declarations: &mut Vec<ApplicableDeclarationBlock>,
+                                  applicable_declarations: &mut ApplicableDeclarationList,
                                   guards: &StylesheetGuards)
                                   -> StrongRuleNode
 {
-    let rules = applicable_declarations.drain(..).map(|d| (d.source, d.level));
+    let rules = applicable_declarations.drain().map(|d| (d.source, d.level));
     let rule_node = rule_tree.insert_ordered_rules_with_important(rules, guards);
     rule_node
 }
@@ -994,8 +994,7 @@ pub trait MatchMethods : TElement {
             }
         }
 
-        let mut applicable_declarations =
+        let mut applicable_declarations = ApplicableDeclarationList::new();
-            Vec::<ApplicableDeclarationBlock>::with_capacity(16);
 
         let stylist = &context.shared.stylist;
         let style_attribute = self.style_attribute();
@@ -1054,8 +1053,7 @@ pub trait MatchMethods : TElement {
             return false;
         }
 
-        let mut applicable_declarations =
+        let mut applicable_declarations = ApplicableDeclarationList::new();
-            Vec::<ApplicableDeclarationBlock>::with_capacity(16);
 
         let map = &mut context.thread_local.selector_flags;
         let mut set_selector_flags = |element: &Self, flags: ElementSelectorFlags| {

components/style/stylist.rs

@@ -33,7 +33,7 @@ use selectors::parser::{SelectorMethods, LocalName as LocalNameSelector};
 use selectors::visitor::SelectorVisitor;
 use shared_lock::{Locked, SharedRwLockReadGuard, StylesheetGuards};
 use sink::Push;
-use smallvec::VecLike;
+use smallvec::{SmallVec, VecLike};
 use std::borrow::Borrow;
 use std::collections::HashMap;
 use std::hash::Hash;
@@ -49,6 +49,15 @@ use viewport::{self, MaybeNew, ViewportRule};
 
 pub use ::fnv::FnvHashMap;
 
+/// List of applicable declaration. This is a transient structure that shuttles
+/// declarations between selector matching and inserting into the rule tree, and
+/// therefore we want to avoid heap-allocation where possible.
+///
+/// In measurements on wikipedia, we pretty much never have more than 8 applicable
+/// declarations, so we could consider making this 8 entries instead of 16.
+/// However, it may depend a lot on workload, and stack space is cheap.
+pub type ApplicableDeclarationList = SmallVec<[ApplicableDeclarationBlock; 16]>;
+
 /// This structure holds all the selectors and device characteristics
 /// for a given document. The selectors are converted into `Rule`s
 /// (defined in rust-selectors), and introduced in a `SelectorMap`
@@ -678,7 +687,7 @@ impl Stylist {
         };
 
 
-        let mut declarations = vec![];
+        let mut declarations = ApplicableDeclarationList::new();
         self.push_applicable_declarations(element,
                                           None,
                                           None,