FieldValue and DocumentReference codable support. (#3231)

* working serverTimestamp

* working FieldValue. Still need: document ref and ability to Eq of FieldValue

* DocumentReference implemented and testing works

* clean up header search path

* improving FieldValue enum comments

* run through formatting

* addressing comments

* formatted with swiftformat 0.35.7

* add missing bool->BOOL change

* undo unintended swift version change

* remove unwanted proj file change

Author: Hui-Wu

Firestore/Swift/Source/Codable/third_party/FirestoreDecoder.swift

@@ -1033,7 +1033,7 @@ extension _FirestoreDecoder {
       }
     }
 
-    // Decoding an embeded container, this requires expanding the storage stack and
+    // Decoding an embedded container, this requires expanding the storage stack and
     // then restore after decoding.
     storage.push(container: value)
     let decoded = try T(from: self)

Firestore/Swift/Source/Codable/third_party/FirestoreEncoder.swift

@@ -35,6 +35,10 @@ extension Firestore {
     /// - Returns: A Map keyed by String representing a document Firestore
     ///            API can work with.
     public func encode<T: Encodable>(_ value: T) throws -> [String: Any] {
+      // DocumentReference and FieldValue cannot be encoded directly.
+      guard T.self != DocumentReference.self && T.self != FieldValue.self else {
+        throw FirestoreEncodingError.encodingIsNotSupported
+      }
       guard let topLevel = try _FirestoreEncoder().box_(value) else {
         throw EncodingError.invalidValue(value,
                                          EncodingError.Context(codingPath: [],
@@ -495,7 +499,7 @@ extension _FirestoreEncoder: SingleValueEncodingContainer {
 }
 
 /// Special subclass of `_FirestoreEncoder` used by `superEncoder`.
-/// It inherits the codingPath from the referencing `_FirestoreEncoder` but uses it's own
+/// It inherits the codingPath from the referencing `_FirestoreEncoder` but uses its own
 /// storage. The encoded result will be written back to the referencing encoder's storage
 /// when it is `deinit`-ed.
 private class _FirestoreReferencingEncoder: _FirestoreEncoder {

Firestore/third_party/FirestoreEncoder/FirestoreDecoder.swift

@@ -1033,7 +1033,7 @@ extension _FirestoreDecoder {
       }
     }
 
-    // Decoding an embeded container, this requires expanding the storage stack and
+    // Decoding an embedded container, this requires expanding the storage stack and
     // then restore after decoding.
     storage.push(container: value)
     let decoded = try T(from: self)

Firestore/third_party/FirestoreEncoder/FirestoreEncoder.swift

@@ -35,6 +35,10 @@ extension Firestore {
     /// - Returns: A Map keyed by String representing a document Firestore
     ///            API can work with.
     public func encode<T: Encodable>(_ value: T) throws -> [String: Any] {
+      // DocumentReference and FieldValue cannot be encoded directly.
+      guard T.self != DocumentReference.self && T.self != FieldValue.self else {
+        throw FirestoreEncodingError.encodingIsNotSupported
+      }
       guard let topLevel = try _FirestoreEncoder().box_(value) else {
         throw EncodingError.invalidValue(value,
                                          EncodingError.Context(codingPath: [],
@@ -495,7 +499,7 @@ extension _FirestoreEncoder: SingleValueEncodingContainer {
 }
 
 /// Special subclass of `_FirestoreEncoder` used by `superEncoder`.
-/// It inherits the codingPath from the referencing `_FirestoreEncoder` but uses it's own
+/// It inherits the codingPath from the referencing `_FirestoreEncoder` but uses its own
 /// storage. The encoded result will be written back to the referencing encoder's storage
 /// when it is `deinit`-ed.
 private class _FirestoreReferencingEncoder: _FirestoreEncoder {