Add -Ybackend-parallelism and -release

src/main/scala/io/github/davidgregory084/ScalaVersion.scala

@@ -28,6 +28,7 @@ object ScalaVersion {
   val V2_11_11 = ScalaVersion(2, 11, 11)
   val V2_12_0  = ScalaVersion(2, 12, 0)
   val V2_12_2  = ScalaVersion(2, 12, 2)
+  val V2_12_5  = ScalaVersion(2, 12, 5)
   val V2_13_0  = ScalaVersion(2, 13, 0)
   val V2_13_2  = ScalaVersion(2, 13, 2)
   val V2_13_3  = ScalaVersion(2, 13, 3)

src/main/scala/io/github/davidgregory084/ScalacOptions.scala

@@ -39,6 +39,14 @@ trait ScalacOptions {
   val feature =
     new ScalacOption(List("-feature"))
 
+  /** Compile for a specific version of the Java platform. Supported targets: 8, 9, ..., 17, 18.
+    *
+    * The release flag is supported only on JDK 9 and above, since it relies on the functionality
+    * provided in [[http://openjdk.java.net/jeps/247 JEP-247: Compile for Older Platform Versions]].
+    */
+  def release(version: String) =
+    new ScalacOption(List("-release", version), version => version > V2_12_5)
+
   /** Enable features that will be available in a future version of Scala, for purposes of early
     * migration and alpha testing.
     */
@@ -344,6 +352,15 @@ trait ScalacOptions {
   def privateOption(name: String, isSupported: ScalaVersion => Boolean = _ => true) =
     new ScalacOption(List(s"-Y$name"), isSupported)
 
+  /** Private options (-Y)
+    */
+  def privateOption(
+    name: String,
+    additionalTokens: List[String],
+    isSupported: ScalaVersion => Boolean
+  ) =
+    new ScalacOption(s"-Y$name" :: additionalTokens, isSupported)
+
   /** Produce an error if an argument list is modified to match the receiver.
     */
   val privateNoAdaptedArgs =
@@ -357,13 +374,31 @@ trait ScalacOptions {
 
   /** Enables support for higher order unification in type constructor inference.
     *
-    * Initially provided as a compiler option in the 2.12.x series to fix the infamous [[https://github.com/scala/bug/issues/2712 SI-2712]].
+    * Initially provided as a compiler option in the 2.12.x series to fix the infamous
+    * [[https://github.com/scala/bug/issues/2712 SI-2712]].
     *
     * Enabled by default in 2.13.0+ and no longer accepted by the compiler as an option.
     */
   val privatePartialUnification =
     privateOption("partial-unification", version => version.isBetween(V2_11_11, V2_13_0))
 
+  /** Configures the number of worker threads for the compiler backend.
+    *
+    * As of 2.12.5 the compiler can serialize bytecode, perform method-local optimisations and write
+    * class files in parallel.
+    *
+    * @param threads
+    *   the number of worker threads. Default: 8, or the value returned by
+    *   [[java.lang.Runtime#availableProcessors]] if fewer than 8.
+    */
+  def privateBackendParallelism(
+    threads: Int = math.min(Runtime.getRuntime.availableProcessors, 8)
+  ) = privateOption(
+    "backend-parallelism",
+    List(threads.toString),
+    version => version.isBetween(V2_12_5, V3_0_0)
+  )
+
   /** Private warning options (-Ywarn)
     */
   def privateWarnOption(name: String, isSupported: ScalaVersion => Boolean = _ => true) =

src/main/scala/io/github/davidgregory084/TpolecatPlugin.scala

@@ -125,11 +125,10 @@ object TpolecatPlugin extends AutoPlugin {
   override def projectSettings: Seq[Setting[_]] = Seq(
     Def.derive(
       scalacOptions := {
-        val previous   = scalacOptions.value
-        val scalaV     = scalaVersion.value
-        val filters    = scalacOptionsFor(scalaV, tpolecatExcludeOptions.value).toSet
-        val newOptions = scalacOptionsFor(scalaV, tpolecatScalacOptions.value)
-        (previous ++ newOptions).filterNot(filters).distinct
+        val pluginOptions = tpolecatScalacOptions.value
+        val pluginExcludes = tpolecatExcludeOptions.value
+        val selectedOptions = pluginOptions.diff(pluginExcludes)
+        scalacOptionsFor(scalaVersion.value, selectedOptions)
       }
     ),
     Def.derive(

src/sbt-test/sbt-tpolecat/scalacOptions/build.sbt

@@ -20,7 +20,11 @@ tpolecatDevModeOptions ++= Set(
   ScalacOptions.source3Migration
 )
 
-tpolecatReleaseModeOptions ++= ScalacOptions.optimizerOptions("**")
+tpolecatReleaseModeOptions ++= {
+  ScalacOptions.optimizerOptions("**") +
+    ScalacOptions.release("8") +
+    ScalacOptions.privateBackendParallelism(8)
+}
 
 val Scala211Options =
   Seq(
@@ -225,17 +229,25 @@ TaskKey[Unit]("checkReleaseMode") := {
         "-Xfatal-warnings",
         "-opt:l:method",
         "-opt:l:inline",
-        "-opt-inline-from:**"
+        "-opt-inline-from:**",
+        "-release",
+        "8",
+        "-Ybackend-parallelism",
+        "8"
       )
     case Scala213 =>
       Scala213Options ++ Seq(
         "-Xfatal-warnings",
         "-opt:l:method",
         "-opt:l:inline",
-        "-opt-inline-from:**"
+        "-opt-inline-from:**",
+        "-release",
+        "8",
+        "-Ybackend-parallelism",
+        "8"
       )
-    case Scala30 => Scala30Options ++ Seq("-Xfatal-warnings")
-    case Scala31 => Scala31Options ++ Seq("-Xfatal-warnings")
+    case Scala30 => Scala30Options ++ Seq("-Xfatal-warnings", "-release", "8")
+    case Scala31 => Scala31Options ++ Seq("-Xfatal-warnings", "-release", "8")
   }
 
   val actualOptions = scalacOptions.value
@@ -266,13 +278,3 @@ TaskKey[Unit]("checkThisProjectScalacOptions") := {
   val options = (Compile / scalacOptions).value
   assert(options.contains("non-existent-key"), "Scope ThisProject was ignored")
 }
-
-addCommandAlias(
-  "addScalacOptionsToThisBuild",
-  "set ThisBuild / scalacOptions += \"non-existent-key-2\""
-)
-
-TaskKey[Unit]("checkThisBuildScalacOptions") := {
-  val options = (Compile / scalacOptions).value
-  assert(options.contains("non-existent-key-2"), "Scope ThisBuild was ignored")
-}

src/sbt-test/sbt-tpolecat/scalacOptions/test

@@ -15,9 +15,6 @@
 > +compile
 # Check console options
 > +checkConsoleScalacOptions
-# Check ThisProject
+# Check user can still append their own scalacOptions
 > addScalacOptionsToThisProject
 > +checkThisProjectScalacOptions
-# Check ThisBuild
-> addScalacOptionsToThisBuild
-> +checkThisBuildScalacOptions