style: Convert FnvHash{Set,Map} instances to FxHash{Set,Map}.

Bug: 1477628
Reviewed-by: heycam

components/selectors/Cargo.toml

@@ -25,6 +25,7 @@ matches = "0.1"
 cssparser = "0.24.0"
 log = "0.4"
 fnv = "1.0"
+fxhash = "0.2"
 phf = "0.7.18"
 precomputed-hash = "0.1"
 servo_arc = { version = "0.1", path = "../servo_arc" }

components/selectors/bloom.rs

@@ -297,6 +297,9 @@ impl Clone for BloomStorageBool {
 }
 
 fn hash<T: Hash>(elem: &T) -> u32 {
+    // We generally use FxHasher in Stylo because it's faster than FnvHasher,
+    // but the increased collision rate has outsized effect on the bloom
+    // filter, so we use FnvHasher instead here.
     let mut hasher = FnvHasher::default();
     elem.hash(&mut hasher);
     let hash: u64 = hasher.finish();

components/selectors/lib.rs

@@ -10,6 +10,7 @@ extern crate bitflags;
 #[macro_use]
 extern crate cssparser;
 extern crate fnv;
+extern crate fxhash;
 #[macro_use]
 extern crate log;
 #[macro_use]

components/selectors/nth_index_cache.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-use fnv::FnvHashMap;
+use fxhash::FxHashMap;
 use tree::OpaqueElement;
 
 /// A cache to speed up matching of nth-index-like selectors.
@@ -32,7 +32,7 @@ impl NthIndexCache {
 
 /// The concrete per-pseudo-class cache.
 #[derive(Default)]
-pub struct NthIndexCacheInner(FnvHashMap<OpaqueElement, i32>);
+pub struct NthIndexCacheInner(FxHashMap<OpaqueElement, i32>);
 
 impl NthIndexCacheInner {
     /// Does a lookup for a given element in the cache.