Fix ObjectLens compatibility

build.sbt

@@ -152,7 +152,7 @@ lazy val compilerPlugin = (projectMatrix in file("compiler-plugin"))
     mimaBinaryIssueFilters := Seq(
       ProblemFilters.exclude[ReversedMissingMethodProblem]("scalapb.options.*"),
       ProblemFilters.exclude[DirectMissingMethodProblem]("scalapb.compiler.GeneratorParams.*"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("scalapb.gen.*"),
+      ProblemFilters.exclude[DirectMissingMethodProblem]("scalapb.gen.*")
     ),
     PB.protocVersion := protobufCompilerVersion,
     Compile / PB.targets := Seq(

compiler-plugin/src/main/scala/scalapb/compiler/ProtobufGenerator.scala

@@ -944,7 +944,7 @@ class ProtobufGenerator(
 
     printer
       .add(
-        s"implicit class ${className}Lens[UpperPB](_l: _root_.scalapb.lenses.Lens[UpperPB, ${message.scalaType.fullName}]) extends _root_.scalapb.lenses.ObjectLens[UpperPB, ${message.scalaType.fullName}](_l) {"
+        s"implicit class ${className}Lens[UpperPB](_l: _root_.scalapb.lenses.Lens[UpperPB, ${message.scalaType.fullName}]) extends ${C.MessageLens}[UpperPB, ${message.scalaType.fullName}](_l) {"
       )
       .indent
       .print(message.fields) { case (printer, field) =>
@@ -1470,9 +1470,6 @@ class ProtobufGenerator(
       .when(javaConverterImport)(
         _.add(s"import _root_.scalapb.internal.compat.JavaConverters.${C.WildCardImport}").add()
       )
-      .when(params.lenses && params.scala3Sources)(
-        _.add("import scala.language.adhocExtensions").add()
-      )
       .print(file.scalaOptions.getImportList.asScala) { case (printer, i) =>
         printer.add(s"import $i")
       }
@@ -1730,6 +1727,10 @@ private class Constants(params: GeneratorParams) extends ConstantsBase {
     if (params.scala3Sources) "private" else "private[this]"
 
   val WithOperator: String = if (params.scala3Sources) " & " else " with "
+
+  val MessageLens =
+    if (params.scala3Sources) "_root_.scalapb.lenses.MessageLens"
+    else "_root_.scalapb.lenses.ObjectLens"
 }
 
 object ProtobufGenerator {

lenses/src/main/scala/scalapb/lenses/Lenses.scala

@@ -111,7 +111,6 @@ object Lens extends CompatLensImplicits {
   }
 }
 
-/** Represents a lens that has sub-lenses. */
 class ObjectLens[U, Container](self: Lens[U, Container]) extends Lens[U, Container] {
 
   /** Creates a sub-lens */
@@ -129,6 +128,11 @@ class ObjectLens[U, Container](self: Lens[U, Container]) extends Lens[U, Contain
     u => set(ms.foldLeft[Container](get(u))((p, m) => m(Lens.unit[Container])(p)))(u)
 }
 
+// In Scala 3 source mode, message lenses extend this class to avoid an error of extending
+// non-open class.
+// TODO(thesamet): unify ObjectLens and MessageLens in next major release (#1581)
+abstract class MessageLens[U, Container](self: Lens[U, Container]) extends ObjectLens(self)
+
 trait Updatable[A] extends Any {
   self: A =>
   def update(ms: (Lens[A, A] => Mutation[A])*): A =