Add clang-tidy checks for Firestore (#1078)

* Add a .clang-tidy configuration for Firestore C++

* Fix clang-tidy warnings

  * typedef -> using
  * const ref + rvalue ref -> pass by value
  * NULL -> nullptr
  * remove useless default initializations
  * remove useless const value-type parameter declarations (definitions
    can still use them)
  * use auto instead of repeating types in a cast

* Fix typos

* Address use of static method through instance warnings

* Address use after move warnings

6 files changed, 8 insertions, 7 deletions

diff --git a/Firestore/core/src/firebase/firestore/auth/credentials_provider.h b/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
index 1aa76df..0a1930a 100644
--- a/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
+++ b/Firestore/core/src/firebase/firestore/auth/credentials_provider.h
@@ -31,10 +31,10 @@ namespace firestore {
 namespace auth {
 
 // `TokenErrorListener` is a listener that gets a token or an error.
-typedef std::function<void(util::StatusOr<Token>)> TokenListener;
+using TokenListener = std::function<void(util::StatusOr<Token>)>;
 
 // Listener notified with a User change.
-typedef std::function<void(User user)> UserChangeListener;
+using UserChangeListener = std::function<void(User user)>;
 
 /**
  * Provides methods for getting the uid and token for the current user and
diff --git a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
index 66c3c87..0e1da31 100644
--- a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
+++ b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.h
@@ -78,7 +78,7 @@ class FirebaseCredentialsProvider : public CredentialsProvider {
    */
   struct Contents {
     Contents(FIRApp* app, User&& user)
-        : app(app), current_user(std::move(user)), mutex() {
+        : app(app), current_user(std::move(user)) {
     }
 
     const FIRApp* app;
diff --git a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
index 2bd3acc..ff2d5f7 100644
--- a/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
+++ b/Firestore/core/src/firebase/firestore/auth/firebase_credentials_provider_apple.mm
@@ -24,6 +24,7 @@
 #include "Firestore/core/src/firebase/firestore/util/string_apple.h"
 
 // NB: This is also defined in Firestore/Source/Public/FIRFirestoreErrors.h
+// NOLINTNEXTLINE: public constant
 NSString* const FIRFirestoreErrorDomain = @"FIRFirestoreErrorDomain";
 
 namespace firebase {
@@ -47,7 +48,7 @@ FirebaseCredentialsProvider::FirebaseCredentialsProvider(FIRApp* app)
                 std::unique_lock<std::mutex> lock(contents->mutex);
                 NSDictionary<NSString*, id>* user_info = notification.userInfo;
 
-                // ensure we're only notifiying for the current app.
+                // ensure we're only notifying for the current app.
                 FIRApp* notified_app =
                     user_info[FIRAuthStateDidChangeInternalNotificationAppKey];
                 if (![contents->app isEqual:notified_app]) {
diff --git a/Firestore/core/src/firebase/firestore/auth/token.h b/Firestore/core/src/firebase/firestore/auth/token.h
index 4b2f3aa..56084f8 100644
--- a/Firestore/core/src/firebase/firestore/auth/token.h
+++ b/Firestore/core/src/firebase/firestore/auth/token.h
@@ -42,7 +42,7 @@ namespace auth {
 // TODO(zxu123): Make this support token-type for desktop workflow.
 class Token {
  public:
-  Token(const absl::string_view token, const User& user);
+  Token(absl::string_view token, const User& user);
 
   /** The actual raw token. */
   const std::string& token() const {
diff --git a/Firestore/core/src/firebase/firestore/auth/user.cc b/Firestore/core/src/firebase/firestore/auth/user.cc
index f442d7b..4793fed 100644
--- a/Firestore/core/src/firebase/firestore/auth/user.cc
+++ b/Firestore/core/src/firebase/firestore/auth/user.cc
@@ -22,7 +22,7 @@ namespace firebase {
 namespace firestore {
 namespace auth {
 
-User::User() : uid_(), is_authenticated_(false) {
+User::User() : is_authenticated_(false) {
 }
 
 User::User(const absl::string_view uid) : uid_(uid), is_authenticated_(true) {
diff --git a/Firestore/core/src/firebase/firestore/auth/user.h b/Firestore/core/src/firebase/firestore/auth/user.h
index 3918c61..cc030cc 100644
--- a/Firestore/core/src/firebase/firestore/auth/user.h
+++ b/Firestore/core/src/firebase/firestore/auth/user.h
@@ -41,7 +41,7 @@ class User {
   User();
 
   /** Construct an authenticated user with the given UID. */
-  explicit User(const absl::string_view uid);
+  explicit User(absl::string_view uid);
 
   const std::string& uid() const {
     return uid_;