replacing Auth/FSTUser by C++ auth implementation (#804)

* replacing Auth/FSTUser by C++ auth implementation

* address changes

28 files changed, 221 insertions, 230 deletions

diff --git a/Firestore/Example/Tests/Local/FSTLevelDBLocalStoreTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBLocalStoreTests.mm
index f71f5c9..97e3c5b 100644
--- a/Firestore/Example/Tests/Local/FSTLevelDBLocalStoreTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLevelDBLocalStoreTests.mm
@@ -18,7 +18,6 @@
 
 #import <XCTest/XCTest.h>
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Local/FSTLevelDB.h"
 
 #import "Firestore/Example/Tests/Local/FSTLocalStoreTests.h"
diff --git a/Firestore/Example/Tests/Local/FSTLevelDBMutationQueueTests.mm b/Firestore/Example/Tests/Local/FSTLevelDBMutationQueueTests.mm
index fe79598..2e24055 100644
--- a/Firestore/Example/Tests/Local/FSTLevelDBMutationQueueTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLevelDBMutationQueueTests.mm
@@ -20,7 +20,6 @@
 #include <leveldb/db.h>
 
 #import "Firestore/Protos/objc/firestore/local/Mutation.pbobjc.h"
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Local/FSTLevelDB.h"
 #import "Firestore/Source/Local/FSTLevelDBKey.h"
 #import "Firestore/Source/Local/FSTWriteGroup.h"
@@ -28,6 +27,7 @@
 #import "Firestore/Example/Tests/Local/FSTMutationQueueTests.h"
 #import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "Firestore/core/src/firebase/firestore/util/ordered_code.h"
 
 NS_ASSUME_NONNULL_BEGIN
@@ -37,6 +37,7 @@ using leveldb::Slice;
 using leveldb::Status;
 using leveldb::WriteOptions;
 using Firestore::StringView;
+using firebase::firestore::auth::User;
 using firebase::firestore::util::OrderedCode;
 
 // A dummy mutation value, useful for testing code that's known to examine only mutation keys.
@@ -69,7 +70,7 @@ std::string MutationLikeKey(StringView table, StringView userID, FSTBatchID batc
 - (void)setUp {
   [super setUp];
   _db = [FSTPersistenceTestHelpers levelDBPersistence];
-  self.mutationQueue = [_db mutationQueueForUser:[[FSTUser alloc] initWithUID:@"user"]];
+  self.mutationQueue = [_db mutationQueueForUser:User("user")];
   self.persistence = _db;
 
   FSTWriteGroup *group = [self.persistence startGroupWithAction:@"Start MutationQueue"];
diff --git a/Firestore/Example/Tests/Local/FSTLocalStoreTests.mm b/Firestore/Example/Tests/Local/FSTLocalStoreTests.mm
index 45d1815..f3493ce 100644
--- a/Firestore/Example/Tests/Local/FSTLocalStoreTests.mm
+++ b/Firestore/Example/Tests/Local/FSTLocalStoreTests.mm
@@ -18,7 +18,6 @@
 
 #import <XCTest/XCTest.h>
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTTimestamp.h"
 #import "Firestore/Source/Local/FSTEagerGarbageCollector.h"
@@ -42,6 +41,10 @@
 #import "Firestore/third_party/Immutable/Tests/FSTImmutableSortedDictionary+Testing.h"
 #import "Firestore/third_party/Immutable/Tests/FSTImmutableSortedSet+Testing.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 /** Creates a document version dictionary mapping the document in @a mutation to @a version. */
@@ -77,7 +80,7 @@ FSTDocumentVersionDictionary *FSTVersionDictionary(FSTMutation *mutation,
   id<FSTGarbageCollector> garbageCollector = [[FSTEagerGarbageCollector alloc] init];
   self.localStore = [[FSTLocalStore alloc] initWithPersistence:persistence
                                               garbageCollector:garbageCollector
-                                                   initialUser:[FSTUser unauthenticatedUser]];
+                                                   initialUser:User::Unauthenticated()];
   [self.localStore start];
 
   _batches = [NSMutableArray array];
@@ -112,7 +115,7 @@ FSTDocumentVersionDictionary *FSTVersionDictionary(FSTMutation *mutation,
   id<FSTGarbageCollector> garbageCollector = [[FSTNoOpGarbageCollector alloc] init];
   self.localStore = [[FSTLocalStore alloc] initWithPersistence:self.localStorePersistence
                                               garbageCollector:garbageCollector
-                                                   initialUser:[FSTUser unauthenticatedUser]];
+                                                   initialUser:User::Unauthenticated()];
   [self.localStore start];
 }
 
diff --git a/Firestore/Example/Tests/Local/FSTMemoryMutationQueueTests.mm b/Firestore/Example/Tests/Local/FSTMemoryMutationQueueTests.mm
index ab7afee..5567078 100644
--- a/Firestore/Example/Tests/Local/FSTMemoryMutationQueueTests.mm
+++ b/Firestore/Example/Tests/Local/FSTMemoryMutationQueueTests.mm
@@ -16,12 +16,15 @@
 
 #import "Firestore/Source/Local/FSTMemoryMutationQueue.h"
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Local/FSTMemoryPersistence.h"
 
 #import "Firestore/Example/Tests/Local/FSTMutationQueueTests.h"
 #import "Firestore/Example/Tests/Local/FSTPersistenceTestHelpers.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::User;
+
 @interface FSTMemoryMutationQueueTests : FSTMutationQueueTests
 @end
 
@@ -35,8 +38,7 @@
   [super setUp];
 
   self.persistence = [FSTPersistenceTestHelpers memoryPersistence];
-  self.mutationQueue =
-      [self.persistence mutationQueueForUser:[[FSTUser alloc] initWithUID:@"user"]];
+  self.mutationQueue = [self.persistence mutationQueueForUser:User("user")];
 }
 
 @end
diff --git a/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm b/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
index 020a0a7..62d30fb 100644
--- a/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
+++ b/Firestore/Example/Tests/Local/FSTMutationQueueTests.mm
@@ -16,7 +16,6 @@
 
 #import "Firestore/Example/Tests/Local/FSTMutationQueueTests.h"
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTTimestamp.h"
 #import "Firestore/Source/Local/FSTEagerGarbageCollector.h"
@@ -28,6 +27,10 @@
 
 #import "Firestore/Example/Tests/Util/FSTHelpers.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTMutationQueueTests
@@ -127,8 +130,7 @@ NS_ASSUME_NONNULL_BEGIN
 
   // Restart the queue so that nextBatchID will be reset.
   [self.mutationQueue shutdown];
-  self.mutationQueue =
-      [self.persistence mutationQueueForUser:[[FSTUser alloc] initWithUID:@"user"]];
+  self.mutationQueue = [self.persistence mutationQueueForUser:User("user")];
 
   FSTWriteGroup *group = [self.persistence startGroupWithAction:@"Start MutationQueue"];
   [self.mutationQueue startWithGroup:group];
diff --git a/Firestore/Example/Tests/SpecTests/FSTSpecTests.mm b/Firestore/Example/Tests/SpecTests/FSTSpecTests.mm
index 3335990..87e3213 100644
--- a/Firestore/Example/Tests/SpecTests/FSTSpecTests.mm
+++ b/Firestore/Example/Tests/SpecTests/FSTSpecTests.mm
@@ -19,7 +19,6 @@
 #import <FirebaseFirestore/FIRFirestoreErrors.h>
 #import <GRPCClient/GRPCCall.h>
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTEventManager.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
@@ -43,6 +42,12 @@
 #import "Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h"
 #import "Firestore/Example/Tests/Util/FSTHelpers.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+
+namespace util = firebase::firestore::util;
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 // Disables all other tests; useful for debugging. Multiple tests can have this tag and they'll all
@@ -327,15 +332,19 @@ static NSString *const kNoIOSTag = @"no-ios";
 }
 
 - (void)doChangeUser:(id)UID {
-  FSTUser *user = [UID isEqual:[NSNull null]] ? [FSTUser unauthenticatedUser]
-                                              : [[FSTUser alloc] initWithUID:UID];
-  [self.driver changeUser:user];
+  if (UID == nil || [UID isEqual:[NSNull null]]) {
+    [self.driver changeUser:User::Unauthenticated()];
+  } else {
+    XCTAssert([UID isKindOfClass:[NSString class]]);
+    [self.driver changeUser:User(UID)];
+  }
 }
 
 - (void)doRestart {
   // Any outstanding user writes should be automatically re-sent, so we want to preserve them
   // when re-creating the driver.
-  FSTOutstandingWriteQueues *outstandingWrites = self.driver.outstandingWrites;
+  FSTOutstandingWriteQueues outstandingWrites = self.driver.outstandingWrites;
+  User currentUser = self.driver.currentUser;
 
   [self.driver shutdown];
 
@@ -347,7 +356,7 @@ static NSString *const kNoIOSTag = @"no-ios";
 
   self.driver = [[FSTSyncEngineTestDriver alloc] initWithPersistence:self.driverPersistence
                                                     garbageCollector:self.garbageCollector
-                                                         initialUser:self.driver.currentUser
+                                                         initialUser:currentUser
                                                    outstandingWrites:outstandingWrites];
   [self.driver start];
 }
diff --git a/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h b/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h
index 46601d7..466a347 100644
--- a/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h
+++ b/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h
@@ -16,16 +16,19 @@
 
 #import <Foundation/Foundation.h>
 
+#include <unordered_map>
+
 #import "Firestore/Source/Core/FSTTypes.h"
 #import "Firestore/Source/Remote/FSTRemoteStore.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 @class FSTDocumentKey;
 @class FSTMutation;
 @class FSTMutationResult;
 @class FSTQuery;
 @class FSTQueryData;
 @class FSTSnapshotVersion;
-@class FSTUser;
 @class FSTViewSnapshot;
 @class FSTWatchChange;
 @protocol FSTGarbageCollector;
@@ -54,7 +57,10 @@ NS_ASSUME_NONNULL_BEGIN
 @end
 
 /** Mapping of user => array of FSTMutations for that user. */
-typedef NSDictionary<FSTUser *, NSArray<FSTOutstandingWrite *> *> FSTOutstandingWriteQueues;
+typedef std::unordered_map<firebase::firestore::auth::User,
+                           NSMutableArray<FSTOutstandingWrite *> *,
+                           firebase::firestore::auth::HashUser>
+    FSTOutstandingWriteQueues;
 
 /**
  * A test driver for FSTSyncEngine that allows simulated event delivery and capture. As much as
@@ -93,8 +99,8 @@ typedef NSDictionary<FSTUser *, NSArray<FSTOutstandingWrite *> *> FSTOutstanding
  */
 - (instancetype)initWithPersistence:(id<FSTPersistence>)persistence
                    garbageCollector:(id<FSTGarbageCollector>)garbageCollector
-                        initialUser:(FSTUser *)initialUser
-                  outstandingWrites:(FSTOutstandingWriteQueues *)outstandingWrites
+                        initialUser:(const firebase::firestore::auth::User &)initialUser
+                  outstandingWrites:(const FSTOutstandingWriteQueues &)outstandingWrites
     NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)init NS_UNAVAILABLE;
@@ -222,7 +228,7 @@ typedef NSDictionary<FSTUser *, NSArray<FSTOutstandingWrite *> *> FSTOutstanding
  * each user, so future receiveWriteAck/Error operations will validate the write sent to the mock
  * datastore matches the next outstanding write for that user.
  */
-- (void)changeUser:(FSTUser *)user;
+- (void)changeUser:(const firebase::firestore::auth::User &)user;
 
 /**
  * Returns all query events generated by the FSTSyncEngine in response to the event injection
@@ -246,10 +252,10 @@ typedef NSDictionary<FSTUser *, NSArray<FSTOutstandingWrite *> *> FSTOutstanding
  * sentWritesCount, but not necessarily, since the FSTRemoteStore limits the number of
  * outstanding writes to the backend at a given time.
  */
-@property(nonatomic, strong, readonly) FSTOutstandingWriteQueues *outstandingWrites;
+@property(nonatomic, assign, readonly) const FSTOutstandingWriteQueues &outstandingWrites;
 
 /** The current user for the FSTSyncEngine; determines which mutation queue is active. */
-@property(nonatomic, strong, readonly) FSTUser *currentUser;
+@property(nonatomic, assign, readonly) const firebase::firestore::auth::User &currentUser;
 
 /** The current set of documents in limbo. */
 @property(nonatomic, strong, readonly)
diff --git a/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.mm b/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.mm
index a4de615..1c7eadf 100644
--- a/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.mm
+++ b/Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.mm
@@ -16,10 +16,11 @@
 
 #import "Firestore/Example/Tests/SpecTests/FSTSyncEngineTestDriver.h"
 
+#include <unordered_map>
+
 #import <FirebaseFirestore/FIRFirestoreErrors.h>
 #import <GRPCClient/GRPCCall.h>
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTEventManager.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
@@ -36,6 +37,11 @@
 #import "Firestore/Example/Tests/Core/FSTSyncEngine+Testing.h"
 #import "Firestore/Example/Tests/SpecTests/FSTMockDatastore.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::HashUser;
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTQueryEvent
@@ -71,35 +77,32 @@ NS_ASSUME_NONNULL_BEGIN
 @property(nonatomic, strong, readonly)
     NSMutableDictionary<FSTQuery *, FSTQueryListener *> *queryListeners;
 
-#pragma mark - Other data structures.
-@property(nonatomic, strong, readwrite) FSTUser *currentUser;
-
 @end
 
 @implementation FSTSyncEngineTestDriver {
   // ivar is declared as mutable.
-  NSMutableDictionary<FSTUser *, NSMutableArray<FSTOutstandingWrite *> *> *_outstandingWrites;
+  std::unordered_map<User, NSMutableArray<FSTOutstandingWrite *> *, HashUser> _outstandingWrites;
+
+  User _currentUser;
 }
 
 - (instancetype)initWithPersistence:(id<FSTPersistence>)persistence
                    garbageCollector:(id<FSTGarbageCollector>)garbageCollector {
   return [self initWithPersistence:persistence
                   garbageCollector:garbageCollector
-                       initialUser:[FSTUser unauthenticatedUser]
-                 outstandingWrites:@{}];
+                       initialUser:User::Unauthenticated()
+                 outstandingWrites:{}];
 }
 
 - (instancetype)initWithPersistence:(id<FSTPersistence>)persistence
                    garbageCollector:(id<FSTGarbageCollector>)garbageCollector
-                        initialUser:(FSTUser *)initialUser
-                  outstandingWrites:(FSTOutstandingWriteQueues *)outstandingWrites {
+                        initialUser:(const User &)initialUser
+                  outstandingWrites:(const FSTOutstandingWriteQueues &)outstandingWrites {
   if (self = [super init]) {
-    // Create mutable copy of outstandingWrites.
-    _outstandingWrites = [NSMutableDictionary dictionary];
-    [outstandingWrites enumerateKeysAndObjectsUsingBlock:^(
-                           FSTUser *user, NSArray<FSTOutstandingWrite *> *writes, BOOL *stop) {
-      _outstandingWrites[user] = [writes mutableCopy];
-    }];
+    // Do a deep copy.
+    for (const auto &pair : outstandingWrites) {
+      _outstandingWrites[pair.first] = [pair.second mutableCopy];
+    }
 
     _events = [NSMutableArray array];
 
@@ -139,9 +142,12 @@ NS_ASSUME_NONNULL_BEGIN
   return self;
 }
 
-- (NSDictionary<FSTUser *, NSMutableArray<FSTOutstandingWrite *> *> *)outstandingWrites {
-  return static_cast<NSDictionary<FSTUser *, NSMutableArray<FSTOutstandingWrite *> *> *>(
-      _outstandingWrites);
+- (const FSTOutstandingWriteQueues &)outstandingWrites {
+  return _outstandingWrites;
+}
+
+- (const User &)currentUser {
+  return _currentUser;
 }
 
 - (void)applyChangedOnlineState:(FSTOnlineState)onlineState {
@@ -200,8 +206,8 @@ NS_ASSUME_NONNULL_BEGIN
   [self.remoteStore enableNetwork];
 }
 
-- (void)changeUser:(FSTUser *)user {
-  self.currentUser = user;
+- (void)changeUser:(const User &)user {
+  _currentUser = user;
   [self.syncEngine userDidChange:user];
 }
 
@@ -309,10 +315,10 @@ NS_ASSUME_NONNULL_BEGIN
 #pragma mark - Helper Methods
 
 - (NSMutableArray<FSTOutstandingWrite *> *)currentOutstandingWrites {
-  NSMutableArray<FSTOutstandingWrite *> *writes = _outstandingWrites[self.currentUser];
+  NSMutableArray<FSTOutstandingWrite *> *writes = _outstandingWrites[_currentUser];
   if (!writes) {
     writes = [NSMutableArray array];
-    _outstandingWrites[self.currentUser] = writes;
+    _outstandingWrites[_currentUser] = writes;
   }
   return writes;
 }
diff --git a/Firestore/Source/API/FIRFirestore.mm b/Firestore/Source/API/FIRFirestore.mm
index 5e978cc..6d2d27b 100644
--- a/Firestore/Source/API/FIRFirestore.mm
+++ b/Firestore/Source/API/FIRFirestore.mm
@@ -51,7 +51,7 @@ extern "C" NSString *const FIRFirestoreErrorDomain = @"FIRFirestoreErrorDomain";
 
 @interface FIRFirestore () {
   /** The actual owned DatabaseId instance is allocated in FIRFirestore. */
-  firebase::firestore::model::DatabaseId _databaseID;
+  DatabaseId _databaseID;
 }
 
 @property(nonatomic, strong) NSString *persistenceKey;
diff --git a/Firestore/Source/Auth/FSTCredentialsProvider.h b/Firestore/Source/Auth/FSTCredentialsProvider.h
index 92d5fdc..230a22d 100644
--- a/Firestore/Source/Auth/FSTCredentialsProvider.h
+++ b/Firestore/Source/Auth/FSTCredentialsProvider.h
@@ -16,16 +16,17 @@
 
 #import <Foundation/Foundation.h>
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 NS_ASSUME_NONNULL_BEGIN
 
 @class FIRApp;
 @class FSTDispatchQueue;
-@class FSTUser;
 
 #pragma mark - FSTGetTokenResult
 
 /**
- * The current FSTUser and the authentication token provided by the underlying authentication
+ * The current User and the authentication token provided by the underlying authentication
  * mechanism. This is the result of calling -[FSTCredentialsProvider getTokenForcingRefresh].
  *
  * ## Portability notes: no TokenType on iOS
@@ -38,11 +39,11 @@ NS_ASSUME_NONNULL_BEGIN
 @interface FSTGetTokenResult : NSObject
 
 - (instancetype)init NS_UNAVAILABLE;
-- (instancetype)initWithUser:(FSTUser *)user
+- (instancetype)initWithUser:(const firebase::firestore::auth::User &)user
                        token:(NSString *_Nullable)token NS_DESIGNATED_INITIALIZER;
 
 /** The user with which the token is associated (used for persisting user state on disk, etc.). */
-@property(nonatomic, nonnull, readonly) FSTUser *user;
+@property(nonatomic, assign, readonly) const firebase::firestore::auth::User &user;
 
 /** The actual raw token. */
 @property(nonatomic, copy, nullable, readonly) NSString *token;
@@ -60,8 +61,8 @@ NS_ASSUME_NONNULL_BEGIN
 typedef void (^FSTVoidGetTokenResultBlock)(FSTGetTokenResult *_Nullable token,
                                            NSError *_Nullable error);
 
-/** Listener block notified with an FSTUser. */
-typedef void (^FSTVoidUserBlock)(FSTUser *user);
+/** Listener block notified with a User. */
+typedef void (^FSTVoidUserBlock)(const firebase::firestore::auth::User &user);
 
 #pragma mark - FSTCredentialsProvider
 
diff --git a/Firestore/Source/Auth/FSTCredentialsProvider.mm b/Firestore/Source/Auth/FSTCredentialsProvider.mm
index 653d7ff..cf045e3 100644
--- a/Firestore/Source/Auth/FSTCredentialsProvider.mm
+++ b/Firestore/Source/Auth/FSTCredentialsProvider.mm
@@ -21,36 +21,53 @@
 #import <GRPCClient/GRPCCall.h>
 
 #import "FIRFirestoreErrors.h"
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTClasses.h"
 #import "Firestore/Source/Util/FSTDispatchQueue.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+
+namespace util = firebase::firestore::util;
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 #pragma mark - FSTGetTokenResult
 
+@interface FSTGetTokenResult () {
+  User _user;
+}
+
+@end
+
 @implementation FSTGetTokenResult
-- (instancetype)initWithUser:(FSTUser *)user token:(NSString *_Nullable)token {
+
+- (instancetype)initWithUser:(const User &)user token:(NSString *_Nullable)token {
   if (self = [super init]) {
     _user = user;
     _token = token;
   }
   return self;
 }
+
+- (const User &)user {
+  return _user;
+}
+
 @end
 
 #pragma mark - FSTFirebaseCredentialsProvider
-@interface FSTFirebaseCredentialsProvider ()
+@interface FSTFirebaseCredentialsProvider () {
+  /** The current user as reported to us via our AuthStateDidChangeListener. */
+  User _currentUser;
+}
 
 @property(nonatomic, strong, readonly) FIRApp *app;
 
 /** Handle used to stop receiving auth changes once userChangeListener is removed. */
 @property(nonatomic, strong, nullable, readwrite) id<NSObject> authListenerHandle;
 
-/** The current user as reported to us via our AuthStateDidChangeListener. */
-@property(nonatomic, strong, nonnull, readwrite) FSTUser *currentUser;
-
 /**
  * Counter used to detect if the user changed while a -getTokenForcingRefresh: request was
  * outstanding.
@@ -67,7 +84,7 @@ NS_ASSUME_NONNULL_BEGIN
   self = [super init];
   if (self) {
     _app = app;
-    _currentUser = [[FSTUser alloc] initWithUID:[self.app getUID]];
+    _currentUser = User([self.app getUID]);
     _userCounter = 0;
 
     // Register for user changes so that we can internally track the current user.
@@ -90,13 +107,13 @@ NS_ASSUME_NONNULL_BEGIN
                       }
 
                       NSString *userID = userInfo[FIRAuthStateDidChangeInternalNotificationUIDKey];
-                      FSTUser *newUser = [[FSTUser alloc] initWithUID:userID];
-                      if (![newUser isEqual:self.currentUser]) {
-                        self.currentUser = newUser;
+                      User newUser = User(userID);
+                      if (newUser != _currentUser) {
+                        _currentUser = newUser;
                         self.userCounter++;
                         FSTVoidUserBlock listenerBlock = self.userChangeListener;
                         if (listenerBlock) {
-                          listenerBlock(self.currentUser);
+                          listenerBlock(_currentUser);
                         }
                       }
                     }
@@ -127,7 +144,7 @@ NS_ASSUME_NONNULL_BEGIN
             completion(nil, cancelError);
           } else {
             FSTGetTokenResult *result =
-                [[FSTGetTokenResult alloc] initWithUser:self.currentUser token:token];
+                [[FSTGetTokenResult alloc] initWithUser:_currentUser token:token];
             completion(result, error);
           }
         };
@@ -142,7 +159,7 @@ NS_ASSUME_NONNULL_BEGIN
       FSTAssert(!_userChangeListener, @"UserChangeListener set twice!");
 
       // Fire initial event.
-      block(self.currentUser);
+      block(_currentUser);
     } else {
       FSTAssert(self.authListenerHandle, @"UserChangeListener removed twice!");
       FSTAssert(_userChangeListener, @"UserChangeListener removed without being set!");
diff --git a/Firestore/Source/Auth/FSTEmptyCredentialsProvider.mm b/Firestore/Source/Auth/FSTEmptyCredentialsProvider.mm
index e78452a..8139d79 100644
--- a/Firestore/Source/Auth/FSTEmptyCredentialsProvider.mm
+++ b/Firestore/Source/Auth/FSTEmptyCredentialsProvider.mm
@@ -16,10 +16,13 @@
 
 #import "Firestore/Source/Auth/FSTEmptyCredentialsProvider.h"
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTDispatchQueue.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 @implementation FSTEmptyCredentialsProvider
@@ -33,7 +36,7 @@ NS_ASSUME_NONNULL_BEGIN
   // Since the user never changes, we just need to fire the initial event and don't need to hang
   // onto the block.
   if (block) {
-    block([FSTUser unauthenticatedUser]);
+    block(User::Unauthenticated());
   }
 }
 
diff --git a/Firestore/Source/Auth/FSTUser.h b/Firestore/Source/Auth/FSTUser.h
deleted file mode 100644
index 68ecc4c..0000000
--- a/Firestore/Source/Auth/FSTUser.h
+++ /dev/null
@@ -1,43 +0,0 @@
-/*
- * Copyright 2017 Google
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#import <Foundation/Foundation.h>
-
-NS_ASSUME_NONNULL_BEGIN
-
-/**
- * Simple wrapper around a nullable UID. Mostly exists to make code more readable and for use as
- * a key in dictionaries (since keys cannot be nil).
- */
-@interface FSTUser : NSObject <NSCopying>
-
-/** Returns an FSTUser with a nil UID. */
-+ (instancetype)unauthenticatedUser;
-
-// Porting note: no GOOGLE_CREDENTIALS or FIRST_PARTY equivalent on iOS, see FSTGetTokenResult for
-// more details.
-
-/** Initializes an FSTUser with the given UID. */
-- (instancetype)initWithUID:(NSString *_Nullable)UID NS_DESIGNATED_INITIALIZER;
-- (instancetype)init NS_UNAVAILABLE;
-
-@property(nonatomic, copy, nullable, readonly) NSString *UID;
-
-@property(nonatomic, assign, readonly, getter=isUnauthenticated) BOOL unauthenticated;
-
-@end
-
-NS_ASSUME_NONNULL_END
diff --git a/Firestore/Source/Auth/FSTUser.mm b/Firestore/Source/Auth/FSTUser.mm
deleted file mode 100644
index 605b4e6..0000000
--- a/Firestore/Source/Auth/FSTUser.mm
+++ /dev/null
@@ -1,68 +0,0 @@
-/*
- * Copyright 2017 Google
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-#import "Firestore/Source/Auth/FSTUser.h"
-
-NS_ASSUME_NONNULL_BEGIN
-
-#pragma mark - FSTUser
-
-@implementation FSTUser
-
-@dynamic unauthenticated;
-
-+ (instancetype)unauthenticatedUser {
-  return [[FSTUser alloc] initWithUID:nil];
-}
-
-- (instancetype)initWithUID:(NSString *_Nullable)UID {
-  if (self = [super init]) {
-    _UID = UID;
-  }
-  return self;
-}
-
-- (BOOL)isEqual:(id)object {
-  if (self == object) {
-    return YES;
-  } else if (![object isKindOfClass:[FSTUser class]]) {
-    return NO;
-  } else {
-    FSTUser *other = object;
-    return (self.isUnauthenticated && other.isUnauthenticated) ||
-           [self.UID isEqualToString:other.UID];
-  }
-}
-
-- (NSUInteger)hash {
-  return [self.UID hash];
-}
-
-- (id)copyWithZone:(nullable NSZone *)zone {
-  return self;  // since FSTUser objects are immutable
-}
-
-- (NSString *)description {
-  return [NSString stringWithFormat:@"<FSTUser uid=%@>", self.UID];
-}
-
-- (BOOL)isUnauthenticated {
-  return self.UID == nil;
-}
-
-@end
-
-NS_ASSUME_NONNULL_END
diff --git a/Firestore/Source/Core/FSTFirestoreClient.mm b/Firestore/Source/Core/FSTFirestoreClient.mm
index 1961460..823f488 100644
--- a/Firestore/Source/Core/FSTFirestoreClient.mm
+++ b/Firestore/Source/Core/FSTFirestoreClient.mm
@@ -38,6 +38,8 @@
 #include "Firestore/core/src/firebase/firestore/model/database_id.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
+namespace util = firebase::firestore::util;
+using firebase::firestore::auth::User;
 using firebase::firestore::core::DatabaseInfo;
 using firebase::firestore::model::DatabaseId;
 
@@ -98,13 +100,15 @@ NS_ASSUME_NONNULL_BEGIN
     _workerDispatchQueue = workerDispatchQueue;
 
     dispatch_semaphore_t initialUserAvailable = dispatch_semaphore_create(0);
-    __block FSTUser *initialUser;
+    __block bool initialized = false;
+    __block User initialUser;
     FSTWeakify(self);
-    _credentialsProvider.userChangeListener = ^(FSTUser *user) {
+    _credentialsProvider.userChangeListener = ^(const User &user) {
       FSTStrongify(self);
       if (self) {
-        if (!initialUser) {
+        if (!initialized) {
           initialUser = user;
+          initialized = true;
           dispatch_semaphore_signal(initialUserAvailable);
         } else {
           [workerDispatchQueue dispatchAsync:^{
@@ -126,7 +130,7 @@ NS_ASSUME_NONNULL_BEGIN
   return self;
 }
 
-- (void)initializeWithUser:(FSTUser *)user usePersistence:(BOOL)usePersistence {
+- (void)initializeWithUser:(const User &)user usePersistence:(BOOL)usePersistence {
   // Do all of our initialization on our own dispatch queue.
   [self.workerDispatchQueue verifyIsCurrentQueue];
 
@@ -189,10 +193,10 @@ NS_ASSUME_NONNULL_BEGIN
   [_remoteStore start];
 }
 
-- (void)userDidChange:(FSTUser *)user {
+- (void)userDidChange:(const User &)user {
   [self.workerDispatchQueue verifyIsCurrentQueue];
 
-  FSTLog(@"User Changed: %@", user);
+  FSTLog(@"User Changed: %@", util::WrapNSStringNoCopy(user.uid()));
   [self.syncEngine userDidChange:user];
 }
 
diff --git a/Firestore/Source/Core/FSTSyncEngine.h b/Firestore/Source/Core/FSTSyncEngine.h
index 7060155..3b28e36 100644
--- a/Firestore/Source/Core/FSTSyncEngine.h
+++ b/Firestore/Source/Core/FSTSyncEngine.h
@@ -19,13 +19,14 @@
 #import "Firestore/Source/Core/FSTTypes.h"
 #import "Firestore/Source/Remote/FSTRemoteStore.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 @class FSTDispatchQueue;
 @class FSTLocalStore;
 @class FSTMutation;
 @class FSTQuery;
 @class FSTRemoteEvent;
 @class FSTRemoteStore;
-@class FSTUser;
 @class FSTViewSnapshot;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -57,7 +58,8 @@ NS_ASSUME_NONNULL_BEGIN
 - (instancetype)init NS_UNAVAILABLE;
 - (instancetype)initWithLocalStore:(FSTLocalStore *)localStore
                        remoteStore:(FSTRemoteStore *)remoteStore
-                       initialUser:(FSTUser *)user NS_DESIGNATED_INITIALIZER;
+                       initialUser:(const firebase::firestore::auth::User &)user
+    NS_DESIGNATED_INITIALIZER;
 
 /**
  * A delegate to be notified when queries being listened to produce new view snapshots or
@@ -98,7 +100,7 @@ NS_ASSUME_NONNULL_BEGIN
                    updateBlock:(FSTTransactionBlock)updateBlock
                     completion:(FSTVoidIDErrorBlock)completion;
 
-- (void)userDidChange:(FSTUser *)user;
+- (void)userDidChange:(const firebase::firestore::auth::User &)user;
 
 /** Applies an FSTOnlineState change to the sync engine and notifies any views of the change. */
 - (void)applyChangedOnlineState:(FSTOnlineState)onlineState;
diff --git a/Firestore/Source/Core/FSTSyncEngine.mm b/Firestore/Source/Core/FSTSyncEngine.mm
index d6ee956..61fac7d 100644
--- a/Firestore/Source/Core/FSTSyncEngine.mm
+++ b/Firestore/Source/Core/FSTSyncEngine.mm
@@ -16,10 +16,11 @@
 
 #import "Firestore/Source/Core/FSTSyncEngine.h"
 
+#include <unordered_map>
+
 #import <GRPCClient/GRPCCall.h>
 
 #import "FIRFirestoreErrors.h"
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
 #import "Firestore/Source/Core/FSTTransaction.h"
@@ -40,8 +41,11 @@
 #import "Firestore/Source/Util/FSTDispatchQueue.h"
 #import "Firestore/Source/Util/FSTLogger.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "Firestore/core/src/firebase/firestore/core/target_id_generator.h"
 
+using firebase::firestore::auth::HashUser;
+using firebase::firestore::auth::User;
 using firebase::firestore::core::TargetIdGenerator;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -139,23 +143,22 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
 /** The garbage collector used to collect documents that are no longer in limbo. */
 @property(nonatomic, strong, readonly) FSTEagerGarbageCollector *limboCollector;
 
-/** Stores user completion blocks, indexed by user and FSTBatchID. */
-@property(nonatomic, strong)
-    NSMutableDictionary<FSTUser *, NSMutableDictionary<NSNumber *, FSTVoidErrorBlock> *>
-        *mutationCompletionBlocks;
-
-@property(nonatomic, strong) FSTUser *currentUser;
-
 @end
 
 @implementation FSTSyncEngine {
   /** Used for creating the FSTTargetIDs for the listens used to resolve limbo documents. */
   TargetIdGenerator _targetIdGenerator;
+
+  /** Stores user completion blocks, indexed by user and FSTBatchID. */
+  std::unordered_map<User, NSMutableDictionary<NSNumber *, FSTVoidErrorBlock> *, HashUser>
+      _mutationCompletionBlocks;
+
+  User _currentUser;
 }
 
 - (instancetype)initWithLocalStore:(FSTLocalStore *)localStore
                        remoteStore:(FSTRemoteStore *)remoteStore
-                       initialUser:(FSTUser *)initialUser {
+                       initialUser:(const User &)initialUser {
   if (self = [super init]) {
     _localStore = localStore;
     _remoteStore = remoteStore;
@@ -169,7 +172,6 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
     _limboDocumentRefs = [[FSTReferenceSet alloc] init];
     [_limboCollector addGarbageSource:_limboDocumentRefs];
 
-    _mutationCompletionBlocks = [NSMutableDictionary dictionary];
     _targetIdGenerator = TargetIdGenerator::SyncEngineTargetIdGenerator(0);
     _currentUser = initialUser;
   }
@@ -227,10 +229,10 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
 
 - (void)addMutationCompletionBlock:(FSTVoidErrorBlock)completion batchID:(FSTBatchID)batchID {
   NSMutableDictionary<NSNumber *, FSTVoidErrorBlock> *completionBlocks =
-      self.mutationCompletionBlocks[self.currentUser];
+      _mutationCompletionBlocks[_currentUser];
   if (!completionBlocks) {
     completionBlocks = [NSMutableDictionary dictionary];
-    self.mutationCompletionBlocks[self.currentUser] = completionBlocks;
+    _mutationCompletionBlocks[_currentUser] = completionBlocks;
   }
   [completionBlocks setObject:completion forKey:@(batchID)];
 }
@@ -400,7 +402,7 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
 
 - (void)processUserCallbacksForBatchID:(FSTBatchID)batchID error:(NSError *_Nullable)error {
   NSMutableDictionary<NSNumber *, FSTVoidErrorBlock> *completionBlocks =
-      self.mutationCompletionBlocks[self.currentUser];
+      _mutationCompletionBlocks[_currentUser];
 
   // NOTE: Mutations restored from persistence won't have completion blocks, so it's okay for
   // this (or the completion below) to be nil.
@@ -527,8 +529,8 @@ static const FSTListenSequenceNumber kIrrelevantSequenceNumber = -1;
   return [self.limboTargetsByKey copy];
 }
 
-- (void)userDidChange:(FSTUser *)user {
-  self.currentUser = user;
+- (void)userDidChange:(const User &)user {
+  _currentUser = user;
 
   // Notify local store and emit any resulting events from swapping out the mutation queue.
   FSTMaybeDocumentDictionary *changes = [self.localStore userDidChange:user];
diff --git a/Firestore/Source/Local/FSTLevelDB.mm b/Firestore/Source/Local/FSTLevelDB.mm
index 7fb3a66..9d3c35e 100644
--- a/Firestore/Source/Local/FSTLevelDB.mm
+++ b/Firestore/Source/Local/FSTLevelDB.mm
@@ -29,11 +29,13 @@
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTLogger.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "Firestore/core/src/firebase/firestore/core/database_info.h"
 #include "Firestore/core/src/firebase/firestore/model/database_id.h"
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
 namespace util = firebase::firestore::util;
+using firebase::firestore::auth::User;
 using firebase::firestore::core::DatabaseInfo;
 using firebase::firestore::model::DatabaseId;
 
@@ -198,7 +200,7 @@ using leveldb::WriteOptions;
 
 #pragma mark - Persistence Factory methods
 
-- (id<FSTMutationQueue>)mutationQueueForUser:(FSTUser *)user {
+- (id<FSTMutationQueue>)mutationQueueForUser:(const User &)user {
   return [FSTLevelDBMutationQueue mutationQueueWithUser:user db:_ptr serializer:self.serializer];
 }
 
diff --git a/Firestore/Source/Local/FSTLevelDBMutationQueue.h b/Firestore/Source/Local/FSTLevelDBMutationQueue.h
index cc05db7..3f1bd51 100644
--- a/Firestore/Source/Local/FSTLevelDBMutationQueue.h
+++ b/Firestore/Source/Local/FSTLevelDBMutationQueue.h
@@ -19,11 +19,12 @@
 #include <memory>
 
 #import "Firestore/Source/Local/FSTMutationQueue.h"
+
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "leveldb/db.h"
 
 @class FSTLevelDB;
 @class FSTLocalSerializer;
-@class FSTUser;
 @protocol FSTGarbageCollector;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -42,7 +43,7 @@ NS_ASSUME_NONNULL_BEGIN
  * @param user The user for which to create a mutation queue.
  * @param db The LevelDB in which to create the queue.
  */
-+ (instancetype)mutationQueueWithUser:(FSTUser *)user
++ (instancetype)mutationQueueWithUser:(const firebase::firestore::auth::User &)user
                                    db:(std::shared_ptr<leveldb::DB>)db
                            serializer:(FSTLocalSerializer *)serializer;
 
diff --git a/Firestore/Source/Local/FSTLevelDBMutationQueue.mm b/Firestore/Source/Local/FSTLevelDBMutationQueue.mm
index 85f163d..982e09c 100644
--- a/Firestore/Source/Local/FSTLevelDBMutationQueue.mm
+++ b/Firestore/Source/Local/FSTLevelDBMutationQueue.mm
@@ -22,7 +22,6 @@
 #include <string>
 
 #import "Firestore/Protos/objc/firestore/local/Mutation.pbobjc.h"
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Local/FSTLevelDB.h"
 #import "Firestore/Source/Local/FSTLevelDBKey.h"
@@ -34,12 +33,15 @@
 #import "Firestore/Source/Model/FSTPath.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 #include "Firestore/core/src/firebase/firestore/util/string_util.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
 namespace util = firebase::firestore::util;
 using Firestore::StringView;
+using firebase::firestore::auth::User;
 using leveldb::DB;
 using leveldb::Iterator;
 using leveldb::ReadOptions;
@@ -89,11 +91,10 @@ static ReadOptions StandardReadOptions() {
   std::shared_ptr<DB> _db;
 }
 
-+ (instancetype)mutationQueueWithUser:(FSTUser *)user
++ (instancetype)mutationQueueWithUser:(const User &)user
                                    db:(std::shared_ptr<DB>)db
                            serializer:(FSTLocalSerializer *)serializer {
-  FSTAssert(![user.UID isEqual:@""], @"UserID must not be an empty string.");
-  NSString *userID = user.isUnauthenticated ? @"" : user.UID;
+  NSString *userID = user.is_authenticated() ? util::WrapNSStringNoCopy(user.uid()) : @"";
 
   return [[FSTLevelDBMutationQueue alloc] initWithUserID:userID db:db serializer:serializer];
 }
diff --git a/Firestore/Source/Local/FSTLocalStore.h b/Firestore/Source/Local/FSTLocalStore.h
index 19803ac..4ec23fd 100644
--- a/Firestore/Source/Local/FSTLocalStore.h
+++ b/Firestore/Source/Local/FSTLocalStore.h
@@ -21,6 +21,8 @@
 #import "Firestore/Source/Model/FSTDocumentKeySet.h"
 #import "Firestore/Source/Model/FSTDocumentVersionDictionary.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 @class FSTLocalViewChanges;
 @class FSTLocalWriteResult;
 @class FSTMutation;
@@ -29,7 +31,6 @@
 @class FSTQuery;
 @class FSTQueryData;
 @class FSTRemoteEvent;
-@class FSTUser;
 @protocol FSTPersistence;
 @protocol FSTGarbageCollector;
 
@@ -80,7 +81,8 @@ NS_ASSUME_NONNULL_BEGIN
 /** Creates a new instance of the FSTLocalStore with its required dependencies as parameters. */
 - (instancetype)initWithPersistence:(id<FSTPersistence>)persistence
                    garbageCollector:(id<FSTGarbageCollector>)garbageCollector
-                        initialUser:(FSTUser *)initialUser NS_DESIGNATED_INITIALIZER;
+                        initialUser:(const firebase::firestore::auth::User &)initialUser
+    NS_DESIGNATED_INITIALIZER;
 
 - (instancetype)init NS_UNAVAILABLE;
 
@@ -96,7 +98,7 @@ NS_ASSUME_NONNULL_BEGIN
  * In response the local store switches the mutation queue to the new user and returns any
  * resulting document changes.
  */
-- (FSTMaybeDocumentDictionary *)userDidChange:(FSTUser *)user;
+- (FSTMaybeDocumentDictionary *)userDidChange:(const firebase::firestore::auth::User &)user;
 
 /** Accepts locally generated Mutations and commits them to storage. */
 - (FSTLocalWriteResult *)locallyWriteMutations:(NSArray<FSTMutation *> *)mutations;
diff --git a/Firestore/Source/Local/FSTLocalStore.mm b/Firestore/Source/Local/FSTLocalStore.mm
index 8a383e5..d12a45b 100644
--- a/Firestore/Source/Local/FSTLocalStore.mm
+++ b/Firestore/Source/Local/FSTLocalStore.mm
@@ -16,7 +16,6 @@
 
 #import "Firestore/Source/Local/FSTLocalStore.h"
 
-#import "Firestore/Source/Auth/FSTUser.h"
 #import "Firestore/Source/Core/FSTListenSequence.h"
 #import "Firestore/Source/Core/FSTQuery.h"
 #import "Firestore/Source/Core/FSTSnapshotVersion.h"
@@ -41,8 +40,10 @@
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTLogger.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
 #include "Firestore/core/src/firebase/firestore/core/target_id_generator.h"
 
+using firebase::firestore::auth::User;
 using firebase::firestore::core::TargetIdGenerator;
 
 NS_ASSUME_NONNULL_BEGIN
@@ -99,7 +100,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 - (instancetype)initWithPersistence:(id<FSTPersistence>)persistence
                    garbageCollector:(id<FSTGarbageCollector>)garbageCollector
-                        initialUser:(FSTUser *)initialUser {
+                        initialUser:(const User &)initialUser {
   if (self = [super init]) {
     _persistence = persistence;
     _mutationQueue = [persistence mutationQueueForUser:initialUser];
@@ -166,7 +167,7 @@ NS_ASSUME_NONNULL_BEGIN
   [self.queryCache shutdown];
 }
 
-- (FSTMaybeDocumentDictionary *)userDidChange:(FSTUser *)user {
+- (FSTMaybeDocumentDictionary *)userDidChange:(const User &)user {
   // Swap out the mutation queue, grabbing the pending mutation batches before and after.
   NSArray<FSTMutationBatch *> *oldBatches = [self.mutationQueue allMutationBatches];
 
diff --git a/Firestore/Source/Local/FSTMemoryPersistence.mm b/Firestore/Source/Local/FSTMemoryPersistence.mm
index e301820..ba71f9c 100644
--- a/Firestore/Source/Local/FSTMemoryPersistence.mm
+++ b/Firestore/Source/Local/FSTMemoryPersistence.mm
@@ -16,7 +16,8 @@
 
 #import "Firestore/Source/Local/FSTMemoryPersistence.h"
 
-#import "Firestore/Source/Auth/FSTUser.h"
+#include <unordered_map>
+
 #import "Firestore/Source/Local/FSTMemoryMutationQueue.h"
 #import "Firestore/Source/Local/FSTMemoryQueryCache.h"
 #import "Firestore/Source/Local/FSTMemoryRemoteDocumentCache.h"
@@ -24,12 +25,15 @@
 #import "Firestore/Source/Local/FSTWriteGroupTracker.h"
 #import "Firestore/Source/Util/FSTAssert.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
+using firebase::firestore::auth::HashUser;
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 @interface FSTMemoryPersistence ()
 @property(nonatomic, strong, nonnull) FSTWriteGroupTracker *writeGroupTracker;
-@property(nonatomic, strong, nonnull)
-    NSMutableDictionary<FSTUser *, id<FSTMutationQueue>> *mutationQueues;
 @property(nonatomic, assign, getter=isStarted) BOOL started;
 @end
 
@@ -46,6 +50,8 @@ NS_ASSUME_NONNULL_BEGIN
 
   /** The FSTRemoteDocumentCache representing the persisted cache of remote documents. */
   FSTMemoryRemoteDocumentCache *_remoteDocumentCache;
+
+  std::unordered_map<User, id<FSTMutationQueue>, HashUser> _mutationQueues;
 }
 
 + (instancetype)persistence {
@@ -57,7 +63,6 @@ NS_ASSUME_NONNULL_BEGIN
     _writeGroupTracker = [FSTWriteGroupTracker tracker];
     _queryCache = [[FSTMemoryQueryCache alloc] init];
     _remoteDocumentCache = [[FSTMemoryRemoteDocumentCache alloc] init];
-    _mutationQueues = [NSMutableDictionary dictionary];
   }
   return self;
 }
@@ -75,11 +80,11 @@ NS_ASSUME_NONNULL_BEGIN
   self.started = NO;
 }
 
-- (id<FSTMutationQueue>)mutationQueueForUser:(FSTUser *)user {
-  id<FSTMutationQueue> queue = self.mutationQueues[user];
+- (id<FSTMutationQueue>)mutationQueueForUser:(const User &)user {
+  id<FSTMutationQueue> queue = _mutationQueues[user];
   if (!queue) {
     queue = [FSTMemoryMutationQueue mutationQueue];
-    self.mutationQueues[user] = queue;
+    _mutationQueues[user] = queue;
   }
   return queue;
 }
diff --git a/Firestore/Source/Local/FSTPersistence.h b/Firestore/Source/Local/FSTPersistence.h
index cf07a9e..30eb211 100644
--- a/Firestore/Source/Local/FSTPersistence.h
+++ b/Firestore/Source/Local/FSTPersistence.h
@@ -16,7 +16,8 @@
 
 #import <Foundation/Foundation.h>
 
-@class FSTUser;
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 @class FSTWriteGroup;
 @protocol FSTMutationQueue;
 @protocol FSTQueryCache;
@@ -75,7 +76,7 @@ NS_ASSUME_NONNULL_BEGIN
  * implementation to the extent possible (e.g. in the case of uid switching from
  * sally=>jack=>sally, sally's mutation queue will be preserved).
  */
-- (id<FSTMutationQueue>)mutationQueueForUser:(FSTUser *)user;
+- (id<FSTMutationQueue>)mutationQueueForUser:(const firebase::firestore::auth::User &)user;
 
 /** Creates an FSTQueryCache representing the persisted cache of queries. */
 - (id<FSTQueryCache>)queryCache;
diff --git a/Firestore/Source/Remote/FSTRemoteStore.h b/Firestore/Source/Remote/FSTRemoteStore.h
index b5dd204..4ea9379 100644
--- a/Firestore/Source/Remote/FSTRemoteStore.h
+++ b/Firestore/Source/Remote/FSTRemoteStore.h
@@ -19,6 +19,8 @@
 #import "Firestore/Source/Core/FSTTypes.h"
 #import "Firestore/Source/Model/FSTDocumentVersionDictionary.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+
 @class FSTDatastore;
 @class FSTDocumentKey;
 @class FSTLocalStore;
@@ -28,7 +30,6 @@
 @class FSTQueryData;
 @class FSTRemoteEvent;
 @class FSTTransaction;
-@class FSTUser;
 
 NS_ASSUME_NONNULL_BEGIN
 
@@ -121,7 +122,7 @@ NS_ASSUME_NONNULL_BEGIN
  * In response the remote store tears down streams and clears up any tracked operations that should
  * not persist across users. Restarts the streams if appropriate.
  */
-- (void)userDidChange:(FSTUser *)user;
+- (void)userDidChange:(const firebase::firestore::auth::User &)user;
 
 /** Listens to the target identified by the given FSTQueryData. */
 - (void)listenToTargetWithQueryData:(FSTQueryData *)queryData;
diff --git a/Firestore/Source/Remote/FSTRemoteStore.mm b/Firestore/Source/Remote/FSTRemoteStore.mm
index 123df49..c6668bf 100644
--- a/Firestore/Source/Remote/FSTRemoteStore.mm
+++ b/Firestore/Source/Remote/FSTRemoteStore.mm
@@ -35,6 +35,12 @@
 #import "Firestore/Source/Util/FSTAssert.h"
 #import "Firestore/Source/Util/FSTLogger.h"
 
+#include "Firestore/core/src/firebase/firestore/auth/user.h"
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+
+namespace util = firebase::firestore::util;
+using firebase::firestore::auth::User;
+
 NS_ASSUME_NONNULL_BEGIN
 
 /**
@@ -268,8 +274,9 @@ static const int kOnlineAttemptsBeforeFailure = 2;
   [self updateOnlineState:FSTOnlineStateUnknown];
 }
 
-- (void)userDidChange:(FSTUser *)user {
-  FSTLog(@"FSTRemoteStore %p changing users: %@", (__bridge void *)self, user);
+- (void)userDidChange:(const User &)user {
+  FSTLog(@"FSTRemoteStore %p changing users: %@", (__bridge void *)self,
+         util::WrapNSStringNoCopy(user.uid()));
   if ([self isNetworkEnabled]) {
     // Tear down and re-create our network streams. This will ensure we get a fresh auth token
     // for the new user and re-fill the write pipeline with new mutations from the LocalStore
diff --git a/Firestore/core/src/firebase/firestore/auth/user.h b/Firestore/core/src/firebase/firestore/auth/user.h
index 58b8b55..cc7b66d 100644
--- a/Firestore/core/src/firebase/firestore/auth/user.h
+++ b/Firestore/core/src/firebase/firestore/auth/user.h
@@ -17,6 +17,11 @@
 #ifndef FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_AUTH_USER_H_
 #define FIRESTORE_CORE_SRC_FIREBASE_FIRESTORE_AUTH_USER_H_
 
+#if defined(__OBJC__)
+#import <Foundation/Foundation.h>
+#include "Firestore/core/src/firebase/firestore/util/string_apple.h"
+#endif  // defined(__OBJC__)
+
 #include <string>
 
 #include "absl/strings/string_view.h"
@@ -38,6 +43,12 @@ class User {
   /** Construct an authenticated user with the given UID. */
   explicit User(const absl::string_view uid);
 
+#if defined(__OBJC__)
+  explicit User(NSString* uid)
+      : User(firebase::firestore::util::MakeStringView(uid)) {
+  }
+#endif  // defined(__OBJC__)
+
   const std::string& uid() const {
     return uid_;
   }
@@ -69,6 +80,14 @@ inline bool operator!=(const User& lhs, const User& rhs) {
   return !(lhs == rhs);
 }
 
+// Specializations of std::hash is prohibited. We define a hash function to be
+// passed through manually.
+struct HashUser {
+  inline int64_t operator()(const User& user) const {
+    return std::hash<std::string>{}(user.uid());
+  }
+};
+
 }  // namespace auth
 }  // namespace firestore
 }  // namespace firebase
diff --git a/Firestore/core/test/firebase/firestore/auth/user_test.cc b/Firestore/core/test/firebase/firestore/auth/user_test.cc
index a9f764d..7277283 100644
--- a/Firestore/core/test/firebase/firestore/auth/user_test.cc
+++ b/Firestore/core/test/firebase/firestore/auth/user_test.cc
@@ -49,6 +49,11 @@ TEST(User, Comparison) {
   EXPECT_NE(User("abc"), User("xyz"));
 }
 
+TEST(User, Hash) {
+  const HashUser hash;
+  EXPECT_EQ(hash(User("abc")), hash(User("abc")));
+}
+
 }  // namespace auth
 }  // namespace firestore
 }  // namespace firebase