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

2 files changed, 5 insertions, 7 deletions

diff --git a/Firestore/core/src/firebase/firestore/remote/serializer.cc b/Firestore/core/src/firebase/firestore/remote/serializer.cc
index 805e7ac..befe032 100644
--- a/Firestore/core/src/firebase/firestore/remote/serializer.cc
+++ b/Firestore/core/src/firebase/firestore/remote/serializer.cc
@@ -294,7 +294,7 @@ void Writer::WriteVarint(uint64_t value) {
  * Note that (despite the return type) this works for bool, enum, int32, int64,
  * uint32 and uint64 proto field types.
  *
- * Note: This is not expected to be called direclty, but rather only via the
+ * Note: This is not expected to be called directly, but rather only via the
  * other Decode* methods (i.e. DecodeBool, DecodeLong, etc)
  *
  * @return The decoded varint as a uint64_t.
diff --git a/Firestore/core/src/firebase/firestore/remote/serializer.h b/Firestore/core/src/firebase/firestore/remote/serializer.h
index 3b2b667..7541ef5 100644
--- a/Firestore/core/src/firebase/firestore/remote/serializer.h
+++ b/Firestore/core/src/firebase/firestore/remote/serializer.h
@@ -72,7 +72,7 @@ class Serializer {
   // TODO(rsgowman): If we never support any output except to a vector, it may
   // make sense to have Serializer own the vector and provide an accessor rather
   // than asking the user to create it first.
-  static util::Status EncodeFieldValue(
+  util::Status EncodeFieldValue(
       const firebase::firestore::model::FieldValue& field_value,
       std::vector<uint8_t>* out_bytes);
 
@@ -84,8 +84,7 @@ class Serializer {
    * @return The model equivalent of the bytes.
    */
   // TODO(rsgowman): error handling.
-  static firebase::firestore::model::FieldValue DecodeFieldValue(
-      const uint8_t* bytes, size_t length);
+  model::FieldValue DecodeFieldValue(const uint8_t* bytes, size_t length);
 
   /**
    * @brief Converts from bytes to the model FieldValue format.
@@ -95,14 +94,13 @@ class Serializer {
    * @return The model equivalent of the bytes.
    */
   // TODO(rsgowman): error handling.
-  static firebase::firestore::model::FieldValue DecodeFieldValue(
-      const std::vector<uint8_t>& bytes) {
+  model::FieldValue DecodeFieldValue(const std::vector<uint8_t>& bytes) {
     return DecodeFieldValue(bytes.data(), bytes.size());
   }
 
  private:
   // TODO(rsgowman): We don't need the database_id_ yet (but will eventually).
-  // const firebase::firestore::model::DatabaseId& database_id_;
+  // model::DatabaseId* database_id_;
 };
 
 }  // namespace remote