Fix unused nowarn in sbt plugins

main/src/main/scala/sbt/plugins/SbtPlugin.scala

@@ -10,11 +10,22 @@ package plugins
 
 import Keys._
 import Def.Setting
+import sbt.SlashSyntax0._
+import sbt.librarymanagement.Configurations.Compile
+import sbt.librarymanagement.{ SemanticSelector, VersionNumber }
 
 object SbtPlugin extends AutoPlugin {
   override def requires = ScriptedPlugin
 
   override lazy val projectSettings: Seq[Setting[_]] = Seq(
-    sbtPlugin := true
+    sbtPlugin := true,
+    Compile / scalacOptions ++= {
+      // silence unused @nowarns in 2.12 because of https://github.com/sbt/sbt/issues/6398
+      // the option is only available since 2.12.13
+      if (VersionNumber(scalaVersion.value).matchesSemVer(SemanticSelector("=2.12 && >=2.12.13")))
+        Some("-Wconf:cat=unused-nowarn:s")
+      else
+        None
+    }
   )
 }