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
+    }
   )
 }

sbt/src/sbt-test/actions/cross-multiproject/build.sbt

@@ -1,5 +1,4 @@
 lazy val scala212 = "2.12.12"
-// keep this at M5 to test full version
 lazy val scala213 = "2.13.1"
 
 ThisBuild / crossScalaVersions := Seq(scala212, scala213)

sbt/src/sbt-test/project/sbt-plugin/build.sbt

@@ -0,0 +1,6 @@
+lazy val root = project.in(file("."))
+  .enablePlugins(SbtPlugin)
+  .settings(
+    scalaVersion := "2.12.13",
+    scalacOptions ++= Seq("-Xfatal-warnings", "-Xlint")
+  )

sbt/src/sbt-test/project/sbt-plugin/src/main/scala/myplugin/MyPlugin.scala

@@ -0,0 +1,22 @@
+package myplugin
+
+import sbt._
+import sbt.Keys._
+
+case object MyPlugin extends AutoPlugin {
+  object autoImport {
+    val helloWorld = Def.taskKey[String]("log and return hello world")
+  }
+  import autoImport._
+  override def projectSettings: Seq[Def.Setting[_]] = Seq(
+    // should not produce a "@nowarn annotation does not suppres any warnings" warning
+    helloWorld := {
+      streams.value.log("Hello world")
+      "Hello world"
+    },
+    Compile / compile := {
+      helloWorld.value // shoult not produce "a pure expression does nothing" warning
+      (Compile / compile).value
+    }
+  )
+}

sbt/src/sbt-test/project/sbt-plugin/test

@@ -0,0 +1 @@
+> compile