Introduce TransparentUnorderedStringMap.

The added file contains a simple alias of an std::unordered_map
that supports looking up std::string_view without incurring a copy.

Bug: 349741884
Change-Id: I50ef7faa2a1c04b03d4ea9c3d9bbb26aff2c6b77
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5676786
Reviewed-by: Colin Blundell <blundell@chromium.org>
Commit-Queue: Jan Keitel <jkeitel@google.com>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1323590}

components/prefs/BUILD.gn

@@ -47,6 +47,7 @@ component("prefs") {
     "scoped_user_pref_update.h",
     "segregated_pref_store.cc",
     "segregated_pref_store.h",
+    "transparent_unordered_string_map.h",
     "value_map_pref_store.cc",
     "value_map_pref_store.h",
     "wrap_with_prefix_pref_store.cc",

components/prefs/pref_service.h

@@ -34,6 +34,7 @@
 #include "components/prefs/persistent_pref_store.h"
 #include "components/prefs/pref_value_store.h"
 #include "components/prefs/prefs_export.h"
+#include "components/prefs/transparent_unordered_string_map.h"
 
 #if BUILDFLAG(IS_ANDROID)
 #include "base/android/scoped_java_ref.h"
@@ -451,11 +452,7 @@ class COMPONENTS_PREFS_EXPORT PrefService {
   // string comparisons. Order is unimportant, and deletions are rare.
   // Confirmed on Android where this speeded Chrome startup by roughly 50ms
   // vs. std::map, and by roughly 180ms vs. std::set of Preference pointers.
-  struct StringViewHasher : public std::hash<std::string_view> {
-    using is_transparent = void;
-  };
-  using PreferenceMap = std::
-      unordered_map<std::string, Preference, StringViewHasher, std::equal_to<>>;
+  using PreferenceMap = TransparentUnorderedStringMap<Preference>;
 
   // Give access to ReportUserPrefChanged() and GetMutableUserPref().
   friend class subtle::ScopedUserPrefUpdateBase;

components/prefs/transparent_unordered_string_map.h

@@ -0,0 +1,28 @@
+// Copyright 2024 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_PREFS_TRANSPARENT_UNORDERED_STRING_MAP_H_
+#define COMPONENTS_PREFS_TRANSPARENT_UNORDERED_STRING_MAP_H_
+
+#include <functional>
+#include <string>
+#include <string_view>
+#include <unordered_map>
+
+namespace internal {
+struct StringViewHasher : public std::hash<std::string_view> {
+  using is_transparent = void;
+};
+}  // namespace internal
+
+// A `std::unordered_map` from `std::string` to `ValueType` that allows
+// copy-less find for `std::string_view`.
+template <typename ValueType>
+using TransparentUnorderedStringMap =
+    std::unordered_map<std::string,
+                       ValueType,
+                       internal::StringViewHasher,
+                       std::equal_to<>>;
+
+#endif  // COMPONENTS_PREFS_TRANSPARENT_UNORDERED_STRING_MAP_H_

components/sync_preferences/pref_model_associator.h

@@ -18,6 +18,7 @@
 #include "base/observer_list.h"
 #include "base/sequence_checker.h"
 #include "base/values.h"
+#include "components/prefs/transparent_unordered_string_map.h"
 #include "components/prefs/writeable_pref_store.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/model/sync_data.h"
@@ -206,13 +207,7 @@ class PrefModelAssociator final : public syncer::SyncableService,
   // from sync.
   using SyncedPrefObserverList =
       base::ObserverList<SyncedPrefObserver>::Unchecked;
-  struct StringViewHasher : public std::hash<std::string_view> {
-    using is_transparent = void;
-  };
-  std::unordered_map<std::string,
-                     std::unique_ptr<SyncedPrefObserverList>,
-                     StringViewHasher,
-                     std::equal_to<>>
+  TransparentUnorderedStringMap<std::unique_ptr<SyncedPrefObserverList>>
       synced_pref_observers_;
 
   SEQUENCE_CHECKER(sequence_checker_);

services/preferences/tracked/pref_hash_filter.h

@@ -21,6 +21,7 @@
 #include "base/memory/scoped_refptr.h"
 #include "base/memory/weak_ptr.h"
 #include "base/values.h"
+#include "components/prefs/transparent_unordered_string_map.h"
 #include "mojo/public/cpp/bindings/pending_remote.h"
 #include "mojo/public/cpp/bindings/remote.h"
 #include "services/preferences/public/mojom/preferences.mojom.h"
@@ -138,14 +139,8 @@ class PrefHashFilter final : public InterceptablePrefFilter {
 
   // A map of paths to TrackedPreferences; this map owns this individual
   // TrackedPreference objects.
-  struct StringViewHasher : public std::hash<std::string_view> {
-    using is_transparent = void;
-  };
   using TrackedPreferencesMap =
-      std::unordered_map<std::string,
-                         std::unique_ptr<TrackedPreference>,
-                         StringViewHasher,
-                         std::equal_to<>>;
+      TransparentUnorderedStringMap<std::unique_ptr<TrackedPreference>>;
 
   // A map from changed paths to their corresponding TrackedPreferences (which
   // aren't owned by this map).