Add partial support to the binary decoding initializer.

Sources/SwiftProtobuf/BinaryDecodingError.swift

@@ -23,4 +23,8 @@ public enum BinaryDecodingError: Error {
     case malformedProtobuf
     /// The data being parsed does not match the type specified in the proto file
     case schemaMismatch
+    /// The message or nested messages definitions have required fields, and the
+    /// binary data did not include values for them. The `partial` support will
+    /// allow this incomplete data to be decoded.
+    case missingRequiredFields
 }

Sources/SwiftProtobuf/BinaryTypeAdditions.swift

@@ -268,7 +268,20 @@ public extension Message {
         return visitor.serializedSize
     }
 
-    init(serializedData data: Data, extensions: ExtensionSet? = nil) throws {
+    /// Initializes the message by decoding the Protocol Buffer binary serialization
+    /// format for this message.
+    ///
+    /// - Parameters:
+    ///   - serializedData: The binary serialization data to decode.
+    ///   - extensions: An `ExtensionSet` to look up and decode any extensions in this
+    ///     message or messages nested within this message's fields.
+    ///   - partial: By default, the binary serialization format requires all `required`
+    ///     fields be present; when `partial` is `false`,
+    ///     `BinaryDecodingError.missingRequiredFields` is thrown if any were missing.
+    ///     When `partial` is `true`, then partial messages are allowed, and
+    ///     `Message.isInitialized` is not checked.
+    /// - Throws: An instance of `BinaryDecodingError` on failure.
+    init(serializedData data: Data, extensions: ExtensionSet? = nil, partial: Bool = false) throws {
         self.init()
         if !data.isEmpty {
             try data.withUnsafeBytes { (pointer: UnsafePointer<UInt8>) in
@@ -277,6 +290,9 @@ public extension Message {
                                  extensions: extensions)
             }
         }
+        if !partial && !isInitialized {
+            throw BinaryDecodingError.missingRequiredFields
+        }
     }
 }
 

Tests/LinuxMain.swift

@@ -781,14 +781,16 @@ extension Test_Required {
             ("test_NestedInProto2_IsInitialized", {try run_test(test:($0 as! Test_Required).test_NestedInProto2_IsInitialized)}),
             ("test_NestedInProto3_IsInitialized", {try run_test(test:($0 as! Test_Required).test_NestedInProto3_IsInitialized)}),
             ("test_map_isInitialized", {try run_test(test:($0 as! Test_Required).test_map_isInitialized)}),
-            ("test_Extensions_isInitialized", {try run_test(test:($0 as! Test_Required).test_Extensions_isInitialized)})
+            ("test_Extensions_isInitialized", {try run_test(test:($0 as! Test_Required).test_Extensions_isInitialized)}),
+            ("test_decodeRequired", {try run_test(test:($0 as! Test_Required).test_decodeRequired)})
         ]
     }
 }
 
 extension Test_SmallRequired {
     static var allTests: [(String, (XCTestCase) throws -> ())] {
         return [
+            ("test_decodeRequired", {try run_test(test:($0 as! Test_SmallRequired).test_decodeRequired)})
         ]
     }
 }