internal: Introduce scalafix (#3328)

Testing scalafix.

Due to scala2/3 compatibilities, we cannot enable some common rules, but
at least scalacenter/scalafix#542 worked

.scalafix.conf

@@ -0,0 +1,12 @@
+rules = [
+  # ExplicitResultTypes,
+  # OrganizeImports,
+  # RemoveUnused,
+  DisableSyntax,
+  LeakingImplicitClassVal,
+  NoValInForComprehension,
+  # ProcedureSyntax, // Scala-2 only
+  // Need to preserve string-interpolation prefix for `s"..."` and `f"..."` for
+  // preservinng the code alignment
+  # RedundantSyntax
+]

airframe-di/src/main/scala/wvlet/airframe/Session.scala

@@ -136,7 +136,7 @@ object Session extends LogSupport {
     *
     * @param session
     */
-  implicit class SessionAccess(val session: Session) extends AnyVal {
+  implicit class SessionAccess(private val session: Session) extends AnyVal {
     def get[A](surface: Surface)(implicit sourceCode: SourceCode): A = session.get[A](surface)(sourceCode)
     def getOrElse[A](surface: Surface, obj: => A)(implicit sourceCode: SourceCode): A =
       session.getOrElse[A](surface, obj)(sourceCode)

airframe-http-codegen/src/main/scala/wvlet/airframe/http/codegen/client/HttpClientGenerator.scala

@@ -43,7 +43,7 @@ object HttpClientGenerator extends LogSupport {
     }
   }
 
-  private[client] implicit class RichSurface(val s: Surface) extends AnyVal {
+  private[client] implicit class RichSurface(private val s: Surface) extends AnyVal {
     def fullTypeName: String = fullTypeNameOf(s)
   }
 

airframe-http-grpc/src/main/scala/wvlet/airframe/http/grpc/GrpcContext.scala

@@ -35,7 +35,7 @@ object GrpcContext {
   private[grpc] val KEY_ACCEPT       = Metadata.Key.of("accept", Metadata.ASCII_STRING_MARSHALLER)
   private[grpc] val KEY_CONTENT_TYPE = Metadata.Key.of("content-type", Metadata.ASCII_STRING_MARSHALLER)
 
-  private[grpc] implicit class RichMetadata(val m: Metadata) extends AnyVal {
+  private[grpc] implicit class RichMetadata(private val m: Metadata) extends AnyVal {
     def accept: String = Option(m.get(KEY_ACCEPT)).getOrElse(RPCEncoding.ApplicationMsgPack)
     def setAccept(s: String): Unit = {
       m.removeAll(KEY_ACCEPT)

airframe-http-grpc/src/main/scala/wvlet/airframe/http/grpc/internal/GrpcServiceBuilder.scala

@@ -36,7 +36,7 @@ import java.util.concurrent.ExecutorService
 /**
   */
 object GrpcServiceBuilder {
-  private implicit class RichMethod(val m: MethodSurface) extends AnyVal {
+  private implicit class RichMethod(private val m: MethodSurface) extends AnyVal {
 
     private def findClientStreamingArg: Option[MethodParameter] = {
       m.args.find(x => classOf[Rx[_]].isAssignableFrom(x.surface.rawType))

airframe-json/src/main/scala/wvlet/airframe/json/package.scala

@@ -29,11 +29,11 @@ package object json {
   // Alias to encode msgpack into JSON strings with airframe-codec
   type Json = String
 
-  implicit class RichJson(val json: Json) extends AnyVal {
+  implicit class RichJson(private val json: Json) extends AnyVal {
     def toJSONValue: JSONValue = JSON.parseAny(json)
   }
 
-  implicit class JSONValueOps(val jsonValue: JSONValue) extends AnyVal {
+  implicit class JSONValueOps(private val jsonValue: JSONValue) extends AnyVal {
     def /(name: String): Seq[JSONValue] = {
       jsonValue match {
         case jsonObject: JSONObject =>
@@ -95,7 +95,7 @@ package object json {
     }
   }
 
-  implicit class JSONValueSeqOps(val jsonValues: Seq[JSONValue]) extends AnyVal {
+  implicit class JSONValueSeqOps(private val jsonValues: Seq[JSONValue]) extends AnyVal {
     def /(name: String): Seq[JSONValue] = {
       jsonValues.flatMap { jsonValue => jsonValue / name }
     }

airframe-rx/src/main/scala/wvlet/airframe/rx/package.scala

@@ -17,7 +17,7 @@ import scala.concurrent.{ExecutionContext, Future}
 /**
   */
 package object rx {
-  implicit class FutureConverter[A](val f: Future[A]) extends AnyVal {
+  implicit class FutureConverter[A](private val f: Future[A]) extends AnyVal {
     def toRx(implicit ec: ExecutionContext): RxOption[A] = Rx.fromFuture(f)(ec)
   }
 }

project/plugin.sbt

@@ -10,6 +10,9 @@ addSbtPlugin("org.portable-scala"  % "sbt-scalajs-crossproject" % "1.3.2")
 addSbtPlugin("com.eed3si9n"        % "sbt-buildinfo"            % "0.11.0")
 addSbtPlugin("org.jetbrains.scala" % "sbt-ide-settings"         % "1.1.2")
 
+// For auto-code rewrite
+addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.11.1")
+
 // For integration testing
 val SBT_AIRFRAME_VERSION = sys.env.getOrElse("SBT_AIRFRAME_VERSION", "23.12.1")
 addSbtPlugin("org.wvlet.airframe" % "sbt-airframe" % SBT_AIRFRAME_VERSION)