chore: Add plugin to copy stdlib files from Scala 2

Author: hamzaremmal

project/Build.scala

@@ -20,6 +20,8 @@ import sbt.Package.ManifestAttributes
 import sbt.PublishBinPlugin.autoImport._
 import dotty.tools.sbtplugin.RepublishPlugin
 import dotty.tools.sbtplugin.RepublishPlugin.autoImport._
+import dotty.tools.sbtplugin.ScalaLibraryPlugin
+
 import sbt.plugins.SbtPlugin
 import sbt.ScriptedPlugin.autoImport._
 import xerial.sbt.Sonatype.autoImport._
@@ -1207,6 +1209,7 @@ object Build {
    *  This version of the library is not (yet) TASTy/binary compatible with the Scala 2 compiled library.
    */
   lazy val `scala2-library-bootstrapped` = project.in(file("scala2-library-bootstrapped")).
+    enablePlugins(ScalaLibraryPlugin).
     withCommonSettings(Bootstrapped).
     dependsOn(dottyCompiler(Bootstrapped) % "provided; compile->runtime; test->test").
     settings(scala2LibraryBootstrappedSettings).

project/ScalaLibraryPlugin.scala

@@ -0,0 +1,108 @@
+package dotty.tools.sbtplugin
+
+import sbt.*
+import sbt.Keys.*
+import scala.jdk.CollectionConverters.*
+import java.nio.file.Files
+
+object ScalaLibraryPlugin extends AutoPlugin {
+
+  override def trigger = noTrigger
+
+  val fetchScala2ClassFiles = taskKey[(Set[File], File)]("Fetch the files to use that were compiled with Scala 2")
+  //val scala2LibraryVersion  = settingKey[String]("Version of the Scala 2 Standard Library")
+
+  override def projectSettings = Seq (
+    fetchScala2ClassFiles := {
+      val stream = streams.value
+      val cache  = stream.cacheDirectory
+      val target = cache / "scala-library-classes"
+      val report = update.value
+
+      val scalaLibraryBinaryJar = report.select(
+        configuration = configurationFilter(),
+        module = (_: ModuleID).name == "scala-library",
+        artifact = artifactFilter(`type` = "jar")).headOption.getOrElse {
+          sys.error(s"Could not fetch scala-library binary JAR")
+        }
+
+      if (!target.exists()) {
+        IO.createDirectory(target)
+      }
+
+      (FileFunction.cached(cache / "fetch-scala-library-classes", FilesInfo.lastModified, FilesInfo.exists) { _ =>
+        stream.log.info(s"Unpacking scala-library binaries to persistent directory: ${target.getAbsolutePath}")
+        IO.unzip(scalaLibraryBinaryJar, target)
+        (target ** "*.class").get.toSet
+      } (Set(scalaLibraryBinaryJar)), target)
+
+    },
+    (Compile / compile) := {
+      val stream = streams.value
+      val target = (Compile / classDirectory).value
+      val (files, reference) = fetchScala2ClassFiles.value;
+      val analysis = (Compile / compile).value
+      stream.log.info(s"Copying files from Scala 2 Standard Library to $target")
+      for (file <- files; id <- file.relativeTo(reference).map(_.toString())) {
+        if (filesToCopy(id)) {
+          stream.log.debug(s"Copying file '${id}' to ${target / id}")
+          IO.copyFile(file, target / id)
+        }
+      }
+
+      val overwrittenBinaries = Files.walk((Compile / classDirectory).value.toPath())
+        .iterator()
+        .asScala
+        .map(_.toFile)
+        .map(_.relativeTo((Compile / classDirectory).value).get)
+        .toSet
+      val diff = files.filterNot(_.relativeTo(reference).exists(overwrittenBinaries))
+
+      IO.copy(diff.map { file =>
+        file -> (Compile / classDirectory).value / file.relativeTo(reference).get.getPath
+      })
+
+      analysis
+    }
+  )
+
+  private lazy val filesToCopy = Set(
+    "scala/Tuple1.class",
+    "scala/Tuple2.class",
+    "scala/collection/DoubleStepper.class",
+    "scala/collection/IntStepper.class",
+    "scala/collection/LongStepper.class",
+    "scala/collection/immutable/DoubleVectorStepper.class",
+    "scala/collection/immutable/IntVectorStepper.class",
+    "scala/collection/immutable/LongVectorStepper.class",
+    "scala/jdk/DoubleAccumulator.class",
+    "scala/jdk/IntAccumulator.class",
+    "scala/jdk/LongAccumulator.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleBinaryOperator.class",
+    "scala/jdk/FunctionWrappers$FromJavaBooleanSupplier.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleConsumer.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoublePredicate.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleSupplier.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleToIntFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleToLongFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntBinaryOperator.class",
+    "scala/jdk/FunctionWrappers$FromJavaDoubleUnaryOperator.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntPredicate.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntConsumer.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntSupplier.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntToDoubleFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntToLongFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaIntUnaryOperator.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongBinaryOperator.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongConsumer.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongPredicate.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongSupplier.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongToDoubleFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongToIntFunction.class",
+    "scala/jdk/FunctionWrappers$FromJavaLongUnaryOperator.class",
+    "scala/collection/ArrayOps$ReverseIterator.class",
+    "scala/runtime/NonLocalReturnControl.class",
+    "scala/util/Sorting.class", "scala/util/Sorting$.class",  // Contains @specialized annotation
+    )
+
+}