Compile stdlib-bootstrapped with -Yscala2-stdlib

Author: nicolasstucki

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -367,6 +367,7 @@ private sealed trait YSettings:
   val YfromTastyIgnoreList: Setting[List[String]] = MultiStringSetting("-Yfrom-tasty-ignore-list", "file", "List of `tasty` files in jar files that will not be loaded when using -from-tasty")
   val YnoExperimental: Setting[Boolean] = BooleanSetting("-Yno-experimental", "Disable experimental language features")
   val YlegacyLazyVals: Setting[Boolean] = BooleanSetting("-Ylegacy-lazy-vals", "Use legacy (pre 3.3.0) implementation of lazy vals")
+  val Yscala2Stdlib: Setting[Boolean] = BooleanSetting("-Yscala2-stdlib", "Used when compiling the Scala 2 standard library")
 
   val YprofileEnabled: Setting[Boolean] = BooleanSetting("-Yprofile-enabled", "Enable profiling.")
   val YprofileDestination: Setting[String] = StringSetting("-Yprofile-destination", "file", "Where to send profiling output - specify a file, default is to the console.", "")

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -153,6 +153,7 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
   assert(moduleRoot.isTerm)
 
   checkVersion(using ictx)
+  checkScala2Stdlib(using ictx)
 
   private val loadingMirror = defn(using ictx) // was: mirrorThatLoaded(classRoot)
 
@@ -239,6 +240,9 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
         " in " + source)
   }
 
+  private def checkScala2Stdlib(using Context): Unit =
+    assert(!ctx.settings.Yscala2Stdlib.value, "No Scala 2 libraries should be unpickled under -Yscala2-stdlib")
+
   /** The `decls` scope associated with given symbol */
   protected def symScope(sym: Symbol): Scope = symScopes.getOrElseUpdate(sym, newScope(0))
 

project/Build.scala

@@ -937,6 +937,7 @@ object Build {
         Seq("-sourcepath", ((Compile/sourceManaged).value / "scala-library-src").toString)
       },
       Compile / doc / scalacOptions += "-Ydocument-synthetic-types",
+      scalacOptions += "-Yscala2-stdlib",
       scalacOptions -= "-Xfatal-warnings",
       ivyConfigurations += SourceDeps.hide,
       transitiveClassifiers := Seq("sources"),