turbo mode

This creates a performance mode that enables experimental or advanced features that might require some debugging by the build user when it doesn't work.

Initially we are putting the layered ClassLoader (`ClassLoaderLayeringStrategy.AllLibraryJars`) behind this flag.

main/src/main/scala/sbt/Defaults.scala

@@ -164,7 +164,6 @@ object Defaults extends BuildCommon {
   private[sbt] lazy val globalJvmCore: Seq[Setting[_]] =
     Seq(
       compilerCache := state.value get Keys.stateCompilerCache getOrElse CompilerCache.fresh,
-      classLoaderLayeringStrategy :== ClassLoaderLayeringStrategy.AllLibraryJars,
       sourcesInBase :== true,
       autoAPIMappings := false,
       apiMappings := Map.empty,
@@ -285,6 +284,7 @@ object Defaults extends BuildCommon {
         val tempDirectory = taskTemporaryDirectory.value
         () => Clean.deleteContents(tempDirectory, _ => false)
       },
+      turbo :== SysProp.turbo,
       useSuperShell := { if (insideCI.value) false else SysProp.supershell },
       progressReports := {
         val progress = useSuperShell.value
@@ -1049,7 +1049,7 @@ object Defaults extends BuildCommon {
       cp,
       forkedParallelExecution = false,
       javaOptions = Nil,
-      strategy = ClassLoaderLayeringStrategy.AllLibraryJars,
+      strategy = ClassLoaderLayeringStrategy.ScalaLibrary,
       projectId = "",
     )
   }
@@ -1072,7 +1072,7 @@ object Defaults extends BuildCommon {
       cp,
       forkedParallelExecution,
       javaOptions = Nil,
-      strategy = ClassLoaderLayeringStrategy.AllLibraryJars,
+      strategy = ClassLoaderLayeringStrategy.ScalaLibrary,
       projectId = "",
     )
   }
@@ -1886,7 +1886,11 @@ object Defaults extends BuildCommon {
         }
         val base = ModuleID(id.groupID, id.name, sv).withCrossVersion(cross)
         CrossVersion(scalaV, binVersion)(base).withCrossVersion(Disabled())
-      }
+      },
+      classLoaderLayeringStrategy := {
+        if (turbo.value) ClassLoaderLayeringStrategy.AllLibraryJars
+        else ClassLoaderLayeringStrategy.ScalaLibrary
+      },
     )
   // build.sbt is treated a Scala source of metabuild, so to enable deprecation flag on build.sbt we set the option here.
   lazy val deprecationSettings: Seq[Setting[_]] =

main/src/main/scala/sbt/Keys.scala

@@ -474,6 +474,7 @@ object Keys {
     def apply(progress: ExecuteProgress[Task]): TaskProgress = new TaskProgress(progress)
   }
   val useSuperShell = settingKey[Boolean]("Enables (true) or disables the super shell.")
+  val turbo = settingKey[Boolean]("Enables (true) or disables optional performance features.")
   // This key can be used to add custom ExecuteProgress instances
   val progressReports = settingKey[Seq[TaskProgress]]("A function that returns a list of progress reporters.").withRank(DTask)
   private[sbt] val postProgressReports = settingKey[Unit]("Internally used to modify logger.").withRank(DTask)

main/src/main/scala/sbt/internal/SysProp.scala

@@ -93,6 +93,8 @@ object SysProp {
     coursierOpt.orElse(notIvyOpt).getOrElse(true)
   }
 
+  def turbo: Boolean = getOrFalse("sbt.turbo")
+
   def taskTimings: Boolean = getOrFalse("sbt.task.timings")
   def taskTimingsOnShutdown: Boolean = getOrFalse("sbt.task.timings.on.shutdown")
   def taskTimingsThreshold: Long = long("sbt.task.timings.threshold", 0L)

sbt/src/sbt-test/classloader-cache/akka-actor-system/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / turbo := true
+
 val akkaTest = (project in file(".")).settings(
   name := "akka-test",
   scalaVersion := "2.12.8",

sbt/src/sbt-test/classloader-cache/jni/build.sbt

@@ -10,6 +10,8 @@ val wrappedTest = taskKey[Unit]("Test with modified java.library.path")
 def wrap(task: InputKey[Unit]): Def.Initialize[Task[Unit]] =
   Def.sequential(appendToLibraryPath, task.toTask(""), dropLibraryPath)
 
+ThisBuild / turbo := true
+
 val root = (project in file(".")).settings(
   scalaVersion := "2.12.8",
   javacOptions ++= Seq("-source", "1.8", "-target", "1.8", "-h",

sbt/src/sbt-test/classloader-cache/library-mismatch/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / turbo := true
+
 val snapshot = (project in file(".")).settings(
   name := "mismatched-libraries",
   scalaVersion := "2.12.8",

sbt/src/sbt-test/classloader-cache/resources/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / turbo := true
+
 resolvers += "Local Maven" at (baseDirectory.value / "libraries" / "foo" / "ivy").toURI.toURL.toString
 
 libraryDependencies += "sbt" %% "foo-lib" % "0.1.0"

sbt/src/sbt-test/classloader-cache/snapshot/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / turbo := true
+
 import java.nio.file.Files
 import java.nio.file.attribute.FileTime
 import scala.collection.JavaConverters._

sbt/src/sbt-test/classloader-cache/utest/build.sbt

@@ -1,3 +1,5 @@
+ThisBuild / turbo := true
+
 val utestTest = (project in file(".")).settings(
   name := "utest-test",
   scalaVersion := "2.12.8",