reinstate PROTO3 Option support with an implicit

docs/protobuf.md

@@ -22,4 +22,4 @@ val copy: Outer = protobufType.from(proto)
 
 Additional `ProtobufField[T]` instances for `Byte`, `Char`, and `Short` are available from `import magnolify.protobuf.unsafe._`. These conversions are unsafe due to potential overflow.
 
-Nullable type `Option[T]` is not supported when `MsgT` is compiled with Protobuf 3 syntax. This is because Protobuf 3 does not offer a way to check if a field was set, and instead returns `0`, `""`, `false`, etc. when it was not.
+By default nullable type `Option[T]` is not supported when `MsgT` is compiled with Protobuf 3 syntax. This is because Protobuf 3 does not offer a way to check if a field was set, and instead returns `0`, `""`, `false`, etc. when it was not. You can enable Protobuf 3 support for `Option[T]` by adding `import magnolify.protobuf.unsafe.Proto3Option._`. However with this, Scala `None`s will become `0/""/false` in Protobuf and come back as `Some(0/""/false)`.

protobuf/src/main/scala/magnolify/protobuf/ProtobufType.scala

@@ -37,10 +37,32 @@ sealed trait ProtobufType[T, MsgT <: Message] extends Converter[T, MsgT, MsgT] {
   def apply(t: T): MsgT = to(t)
 }
 
+sealed trait ProtobufOption {
+  def check(f: ProtobufField.Record[_], syntax: Syntax): Unit
+}
+
+object ProtobufOption {
+  implicit val proto2Option: ProtobufOption = new ProtobufOption {
+    override def check(f: ProtobufField.Record[_], syntax: Syntax): Unit =
+      if (f.hasOptional) {
+        require(
+          syntax == Syntax.PROTO2,
+          "Option[T] support is PROTO2 only, " +
+            "`import magnolify.protobuf.unsafe.Proto3Option._` to enable PROTO3 support"
+        )
+      }
+  }
+
+  private[protobuf] class Proto3Option extends ProtobufOption {
+    override def check(f: ProtobufField.Record[_], syntax: Syntax): Unit = ()
+  }
+}
+
 object ProtobufType {
   implicit def apply[T, MsgT <: Message](implicit
     f: ProtobufField.Record[T],
-    ct: ClassTag[MsgT]
+    ct: ClassTag[MsgT],
+    po: ProtobufOption
   ): ProtobufType[T, MsgT] =
     new ProtobufType[T, MsgT] {
       if (f.hasOptional) {
@@ -50,7 +72,7 @@ object ProtobufType {
           .asInstanceOf[Descriptor]
           .getFile
           .getSyntax
-        require(syntax == Syntax.PROTO2, "Option[T] support is PROTO2 only")
+        po.check(f, syntax)
       }
 
       @transient private var _newBuilder: Method = _

protobuf/src/main/scala/magnolify/protobuf/unsafe/package.scala

@@ -20,4 +20,8 @@ package object unsafe {
   implicit val pfByte = ProtobufField.from[Int](_.toByte)(_.toInt)
   implicit val pfChar = ProtobufField.from[Int](_.toChar)(_.toInt)
   implicit val pfShort = ProtobufField.from[Int](_.toShort)(_.toInt)
+
+  object Proto3Option {
+    implicit val proto3Option: ProtobufOption = new ProtobufOption.Proto3Option
+  }
 }

protobuf/src/test/scala/magnolify/protobuf/test/ProtobufTypeSpec.scala

@@ -51,17 +51,37 @@ object ProtobufTypeSpec extends MagnolifySpec("ProtobufType") {
 
   test[Integers, IntegersP2]
   test[Integers, IntegersP3]
-  // PROTO3 removes the notion of require vs optional fields.
-  // The new singular field returns default value if unset, making it required essentially.
   test[Required, RequiredP2]
   test[Required, SingularP3]
   test[Nullable, NullableP2]
+
+  // PROTO3 removes the notion of require vs optional fields.
+  // By default `Option[T] are not supported`.
   try {
     test[Nullable, SingularP3]
   } catch {
     case e: IllegalArgumentException =>
-      require(e.getMessage == "requirement failed: Option[T] support is PROTO2 only")
+      require(
+        e.getMessage ==
+          "requirement failed: Option[T] support is PROTO2 only, " +
+            "`import magnolify.protobuf.unsafe.Proto3Option._` to enable PROTO3 support"
+      )
+  }
+
+  // Adding `import magnolify.protobuf.unsafe.Proto3Option._` enables PROTO3 `Option[T]` support.
+  // The new singular field returns default value if unset.
+  // Hence `None` round trips back as `Some(false/0/"")`.
+  {
+    import magnolify.protobuf.unsafe.Proto3Option._
+    val eq = Eq.instance[Nullable] { (x, y) =>
+      x.b.getOrElse(false) == y.b.getOrElse(false) &&
+      x.i.getOrElse(0) == y.i.getOrElse(0) &&
+      x.s.getOrElse("") == y.s.getOrElse("")
+    }
+    val arb = implicitly[Arbitrary[Nullable]]
+    test(classTag[Nullable], arb, classTag[SingularP3], ProtobufType[Nullable, SingularP3], eq)
   }
+
   test[Repeated, RepeatedP2]
   test[Repeated, RepeatedP3]
   test[Nested, NestedP2]