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

5 files changed, 7 insertions, 9 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/database_id.h b/Firestore/core/src/firebase/firestore/model/database_id.h
index e1feca9..0c0e0ec 100644
--- a/Firestore/core/src/firebase/firestore/model/database_id.h
+++ b/Firestore/core/src/firebase/firestore/model/database_id.h
@@ -44,8 +44,7 @@ class DatabaseId {
    * @param project_id The project for the database.
    * @param database_id The database in the project to use.
    */
-  DatabaseId(const absl::string_view project_id,
-             const absl::string_view database_id);
+  DatabaseId(absl::string_view project_id, absl::string_view database_id);
 
   const std::string& project_id() const {
     return project_id_;
diff --git a/Firestore/core/src/firebase/firestore/model/document.cc b/Firestore/core/src/firebase/firestore/model/document.cc
index 16548cd..ae59d15 100644
--- a/Firestore/core/src/firebase/firestore/model/document.cc
+++ b/Firestore/core/src/firebase/firestore/model/document.cc
@@ -39,7 +39,7 @@ bool Document::Equals(const MaybeDocument& other) const {
   if (other.type() != Type::Document) {
     return false;
   }
-  const Document& other_doc = static_cast<const Document&>(other);
+  auto& other_doc = static_cast<const Document&>(other);
   return MaybeDocument::Equals(other) &&
          has_local_mutations_ == other_doc.has_local_mutations_ &&
          data_ == other_doc.data_;
diff --git a/Firestore/core/src/firebase/firestore/model/field_mask.h b/Firestore/core/src/firebase/firestore/model/field_mask.h
index a9f509a..b895ab3 100644
--- a/Firestore/core/src/firebase/firestore/model/field_mask.h
+++ b/Firestore/core/src/firebase/firestore/model/field_mask.h
@@ -44,9 +44,7 @@ class FieldMask {
 
   FieldMask(std::initializer_list<FieldPath> list) : fields_{list} {
   }
-  explicit FieldMask(const std::vector<FieldPath>& fields) : fields_{fields} {
-  }
-  explicit FieldMask(std::vector<FieldPath>&& fields)
+  explicit FieldMask(std::vector<FieldPath> fields)
       : fields_{std::move(fields)} {
   }
 
diff --git a/Firestore/core/src/firebase/firestore/model/field_value.cc b/Firestore/core/src/firebase/firestore/model/field_value.cc
index 1a40331..b0519f7 100644
--- a/Firestore/core/src/firebase/firestore/model/field_value.cc
+++ b/Firestore/core/src/firebase/firestore/model/field_value.cc
@@ -38,7 +38,7 @@ namespace {
 /**
  * This deviates from the other platforms that define TypeOrder. Since
  * we already define Type for union types, we use it together with this
- * function to achive the equivalent order of types i.e.
+ * function to achieve the equivalent order of types i.e.
  *     i) if two types are comparable, then they are of equal order;
  *    ii) otherwise, their order is the same as the order of their Type.
  */
@@ -148,7 +148,8 @@ FieldValue& FieldValue::operator=(FieldValue&& value) {
       return *this;
     default:
       // We just copy over POD union types.
-      return *this = value;
+      *this = value;
+      return *this;
   }
 }
 
diff --git a/Firestore/core/src/firebase/firestore/model/field_value.h b/Firestore/core/src/firebase/firestore/model/field_value.h
index c70e332..3c5af9c 100644
--- a/Firestore/core/src/firebase/firestore/model/field_value.h
+++ b/Firestore/core/src/firebase/firestore/model/field_value.h
@@ -163,7 +163,7 @@ class FieldValue {
   /**
    * Switch to the specified type, if different from the current type.
    */
-  void SwitchTo(const Type type);
+  void SwitchTo(Type type);
 
   Type tag_ = Type::Null;
   union {