build: add Scala 3.8 -Wconf suppression rules

project/Jdk9.scala

@@ -53,16 +53,21 @@ object Jdk9 extends AutoPlugin {
       yield (task / sourceDirectory).value / sourceDirectoryName
   }
 
+  private def releaseOption(scalaVer: String): Seq[String] = {
+    if (JdkOptions.isScala3_8Plus(scalaVer)) Seq("-java-output-version", majorVersion.toString)
+    else Seq("-release", majorVersion.toString)
+  }
+
   lazy val compileJdk9Settings = Seq(
     // following the scala-2.12, scala-sbt-1.0, ... convention
     unmanagedSourceDirectories := additionalSourceDirectories.value,
-    scalacOptions := PekkoBuild.DefaultScalacOptions.value ++ Seq("-release", majorVersion.toString),
+    scalacOptions := PekkoBuild.DefaultScalacOptions.value ++ releaseOption(scalaVersion.value),
     javacOptions := PekkoBuild.DefaultJavacOptions ++ Seq("--release", majorVersion.toString))
 
   lazy val testJdk9Settings = Seq(
     // following the scala-2.12, scala-sbt-1.0, ... convention
     unmanagedSourceDirectories := additionalTestSourceDirectories.value,
-    scalacOptions := PekkoBuild.DefaultScalacOptions.value ++ Seq("-release", majorVersion.toString),
+    scalacOptions := PekkoBuild.DefaultScalacOptions.value ++ releaseOption(scalaVersion.value),
     javacOptions := PekkoBuild.DefaultJavacOptions ++ Seq("--release", majorVersion.toString),
     compile := compile.dependsOn(CompileJdk9 / compile).value,
     classpathConfiguration := TestJdk9,

project/JdkOptions.scala

@@ -35,10 +35,23 @@ object JdkOptions extends AutoPlugin {
     // for LevelDB
     "--add-opens=java.base/java.nio=ALL-UNNAMED" :: Nil
 
-  def targetJdkScalacOptions(scalaVersion: String): Seq[String] =
-    Seq("-release", JdkOptions.targetJavaVersion) ++ {
-      if (scalaVersion.startsWith("3.")) Seq(s"-Xtarget:${targetJavaVersion}") else Seq.empty
+  def isScala3_8Plus(scalaVersion: String): Boolean =
+    scalaVersion.startsWith("3.") && {
+      val parts = scalaVersion.split('.')
+      def safeToInt(s: String): Int = try { s.split('-').head.toInt }
+      catch { case _: NumberFormatException => 0 }
+      parts.length >= 2 && (safeToInt(parts(1)) >= 8 || safeToInt(parts(0)) > 3)
     }
 
+  def targetJdkScalacOptions(scalaVersion: String): Seq[String] = {
+    if (isScala3_8Plus(scalaVersion)) {
+      Seq("-java-output-version", JdkOptions.targetJavaVersion)
+    } else {
+      Seq("-release", JdkOptions.targetJavaVersion) ++ {
+        if (scalaVersion.startsWith("3.")) Seq(s"-Xtarget:${targetJavaVersion}") else Seq.empty
+      }
+    }
+  }
+
   val targetJdkJavacOptions = Seq("--release", targetJavaVersion)
 }

project/PekkoDisciplinePlugin.scala

@@ -74,47 +74,78 @@ object PekkoDisciplinePlugin extends AutoPlugin {
     "pekko-stream-tests-tck",
     "pekko-testkit")
 
+  private val scala3Suppressions = Seq(
+    "-Wconf:msg=Implicit parameters should be provided with a .using. clause:s",
+    "-Wconf:msg=is no longer supported for vararg splices:s",
+    "-Wconf:msg=with as a type operator has been deprecated:s",
+    "-Wconf:msg=SerialVersionUID does nothing on a trait:s",
+    "-Wconf:msg=has been deprecated.*use .= uninitialized. instead:s",
+    "-Wconf:msg=trailing.*_.*for eta-expansion is unnecessary:s",
+    "-Wconf:msg=is not declared infix:s",
+    "-Wconf:msg=._. is deprecated for wildcard arguments of types:s",
+    "-Wconf:msg=Ignoring ..this.. qualifier:s",
+    "-Wconf:msg=Unreachable case except for null:s",
+    "-Wconf:msg=Classic remoting is deprecated:s",
+    "-Wconf:msg=Use EventSourcedBehavior:s",
+    "-Wconf:msg=migration-to-pekko-grpc:s")
+
   lazy val defaultScalaOptions = Def.setting(CrossVersion.partialVersion(scalaVersion.value).get match {
-    case (3, _)  => "-Wconf:cat=unused-nowarn:s,cat=other-shadowing:s,any:e"
-    case (2, 13) => "-Wconf:any:e,cat=unused-nowarn:s,cat=other-shadowing:s"
-    case (2, 12) => "-Wconf:cat=unused-nowarn:s,any:e"
+    case (3, _) => Seq("-Wconf:any:e", "-Wconf:cat=unused-nowarn:s", "-Wconf:cat=other-shadowing:s") ++
+      scala3Suppressions
+    case (2, 13) => Seq("-Wconf:any:e", "-Wconf:cat=unused-nowarn:s", "-Wconf:cat=other-shadowing:s")
+    case (2, 12) => Seq("-Wconf:cat=unused-nowarn:s", "-Wconf:any:e")
   })
 
   lazy val nowarnSettings = Seq(
-    Compile / scalacOptions ++= (
-      if (scalaVersion.value.startsWith("3.")) Nil
-      else Seq(defaultScalaOptions.value)
-    ),
-    Test / scalacOptions ++= (
-      if (scalaVersion.value.startsWith("3.")) Nil
-      else Seq(defaultScalaOptions.value)
-    ),
+    Compile / scalacOptions ++= defaultScalaOptions.value,
+    Test / scalacOptions ++= defaultScalaOptions.value,
     Compile / doc / scalacOptions := Seq())
 
   // ignore Scala compile warnings for Java 20+
   lazy val jvmIgnoreWarnings = {
     System.getProperty("java.version").startsWith("2")
   }
 
+  private val scala3DocSuppressions = Seq(
+    "-Wconf:cat=unused:s",
+    "-Wconf:cat=deprecation:s",
+    "-Wconf:cat=unchecked:s") ++ scala3Suppressions
+
+  // cat=unused:s does not match subcategories (unused-imports, unused-locals, etc.)
+  // in Scala 2.13's -Wconf category matching (exact string match, not hierarchical).
+  // Add explicit rules for each subcategory plus message-based patterns as fallback.
+  private val docsScala2Suppressions = Seq(
+    "-Wconf:cat=unused:s",
+    "-Wconf:cat=unused-imports:s",
+    "-Wconf:cat=unused-locals:s",
+    "-Wconf:cat=unused-pat-vars:s",
+    "-Wconf:cat=unused-params:s",
+    "-Wconf:cat=unused-privates:s",
+    "-Wconf:cat=unused-explicits:s",
+    "-Wconf:msg=Unused import:s",
+    "-Wconf:msg=is never used:s",
+    "-Wconf:cat=deprecation:s",
+    "-Wconf:cat=unchecked:s")
+
   /**
    * We are a little less strict in docs
    */
   lazy val docs =
     Seq(
-      Compile / scalacOptions -= defaultScalaOptions.value,
+      Compile / scalacOptions --= defaultScalaOptions.value,
       Compile / scalacOptions ++=
         (CrossVersion.partialVersion(scalaVersion.value).get match {
-          case (3, _)  => Nil
-          case (2, 13) => Seq("-Wconf:any:e,cat=unused:s,cat=deprecation:s,cat=unchecked:s")
-          case (2, 12) => Seq("-Wconf:cat=unused:s,cat=deprecation:s,cat=unchecked:s,any:e")
+          case (3, _)  => scala3DocSuppressions
+          case (2, 13) => docsScala2Suppressions
+          case (2, 12) => docsScala2Suppressions :+ "-Wconf:any:e"
         }),
       Test / scalacOptions --= Seq("-Xlint", "-unchecked", "-deprecation"),
-      Test / scalacOptions -= defaultScalaOptions.value,
+      Test / scalacOptions --= defaultScalaOptions.value,
       Test / scalacOptions ++=
         (CrossVersion.partialVersion(scalaVersion.value).get match {
-          case (3, _)  => Nil
-          case (2, 13) => Seq("-Wconf:any:e,cat=unused:s,cat=deprecation:s,cat=unchecked:s")
-          case (2, 12) => Seq("-Wconf:cat=unused:s,cat=deprecation:s,cat=unchecked:s,any:e")
+          case (3, _)  => scala3DocSuppressions
+          case (2, 13) => "-Wconf:any:e" +: docsScala2Suppressions
+          case (2, 12) => docsScala2Suppressions :+ "-Wconf:any:e"
         }),
       Compile / doc / scalacOptions := Seq())
 

project/SbtMultiJvmPlugin.scala

@@ -84,14 +84,14 @@ object MultiJvmPlugin extends AutoPlugin {
 
   override lazy val projectSettings = multiJvmSettings
 
-  private[this] def noTestsMessage(scoped: ScopedKey[?])(implicit display: Show[ScopedKey[?]]): String =
+  private def noTestsMessage(scoped: ScopedKey[?])(implicit display: Show[ScopedKey[?]]): String =
     "No tests to run for " + display.show(scoped)
 
   lazy val multiJvmSettings: Seq[Def.Setting[?]] =
     inConfig(MultiJvm)(Defaults.configSettings ++ internalMultiJvmSettings)
 
   // https://github.com/sbt/sbt/blob/v0.13.15/main/actions/src/main/scala/sbt/Tests.scala#L296-L298
-  private[this] def showResults(log: Logger, results: Tests.Output, noTestsMessage: => String): Unit =
+  private def showResults(log: Logger, results: Tests.Output, noTestsMessage: => String): Unit =
     TestResultLogger.Default.copy(printNoTests = TestResultLogger.const(_.info(noTestsMessage))).run(log, results, "")
 
   private def internalMultiJvmSettings =