Provide better error messages when encoding is not available (#3953)

* Rewrite FirestoreEncoderTests using assertThat

* Throw invalidValue for use of invalid top-level values

* Provide an message for not supported errors

Author: wilhuff

Firestore/Swift/Source/Codable/CodableErrors.swift

@@ -15,10 +15,10 @@
  */
 
 public enum FirestoreDecodingError: Error {
-  case decodingIsNotSupported
+  case decodingIsNotSupported(String)
   case fieldNameConfict(String)
 }
 
 public enum FirestoreEncodingError: Error {
-  case encodingIsNotSupported
+  case encodingIsNotSupported(String)
 }

Firestore/Swift/Source/Codable/DocumentReference+Codable.swift

@@ -30,15 +30,19 @@ private protocol CodableDocumentReference: Codable {}
 
 /**
  * DocumentReference's codable implmentation will just throw for most
- * encoder/decoder however. It is only meant to be encoded by FirestoreEncoder/FirestoreDecoder.
+ * encoder/decoder however. It is only meant to be encoded by Firestore.Encoder/Firestore.Decoder.
  */
 extension CodableDocumentReference {
   public init(from decoder: Decoder) throws {
-    throw FirestoreDecodingError.decodingIsNotSupported
+    throw FirestoreDecodingError.decodingIsNotSupported(
+      "DocumentReference values can only be decoded with Firestore.Decoder"
+    )
   }
 
   public func encode(to encoder: Encoder) throws {
-    throw FirestoreEncodingError.encodingIsNotSupported
+    throw FirestoreEncodingError.encodingIsNotSupported(
+      "DocumentReference values can only be encoded with Firestore.Encoder"
+    )
   }
 }
 

Firestore/Swift/Source/Codable/FieldValue+Encodable.swift

@@ -19,9 +19,10 @@ import FirebaseFirestore
 /** Extends FieldValue to conform to Encodable. */
 extension FieldValue: Encodable {
   /// Encoding a FieldValue will throw by default unless the encoder implementation
-  /// explicitly handles it, which is what FirestoreEncoder does.
+  /// explicitly handles it, which is what Firestore.Encoder does.
   public func encode(to encoder: Encoder) throws {
-    throw FirestoreEncodingError.encodingIsNotSupported
+    throw FirestoreEncodingError.encodingIsNotSupported(
+      "FieldValue values can only be encoded with Firestore.Encoder")
   }
 }
 

Firestore/Swift/Source/Codable/SelfDocumentId.swift

@@ -17,7 +17,7 @@
 import FirebaseFirestore
 
 /// A value that is populated in Codable objects with a `DocumentReference` by
-/// the FirestoreDecoder when a document is read.
+/// the Firestore.Decoder when a document is read.
 ///
 /// Note that limitations in Swift compiler-generated Codable implementations
 /// prevent using this type wrapped in an Optional. Optional SelfDocumentIDs
@@ -33,7 +33,7 @@ import FirebaseFirestore
 /// another without adjusting the value here.
 ///
 /// NOTE: Trying to encode/decode this type using encoders/decoders other than
-/// FirestoreEncoder leads to an error.
+/// Firestore.Encoder leads to an error.
 public final class SelfDocumentID: Equatable, Codable {
   // MARK: - Initializers
 
@@ -48,11 +48,13 @@ public final class SelfDocumentID: Equatable, Codable {
   // MARK: - `Codable` implemention.
 
   public init(from decoder: Decoder) throws {
-    throw FirestoreDecodingError.decodingIsNotSupported
+    throw FirestoreDecodingError.decodingIsNotSupported(
+      "SelfDocumentID values can only be decoded with Firestore.Decoder")
   }
 
   public func encode(to encoder: Encoder) throws {
-    throw FirestoreEncodingError.encodingIsNotSupported
+    throw FirestoreEncodingError.encodingIsNotSupported(
+      "SelfDocumentID values can only be encoded with Firestore.Encoder")
   }
 
   // MARK: - Properties