Add -Ythrough-tasty to debug from TASTY compilation direct from sources

compiler/src/dotty/tools/dotc/fromtasty/Debug.scala

@@ -0,0 +1,71 @@
+package dotty.tools
+package dotc
+package fromtasty
+
+import scala.util.control.NonFatal
+
+import dotty.tools.io.Directory
+
+import java.nio.file.{Files, Paths}
+
+object Debug {
+  def main(args: Array[String]): Unit = {
+    // Preload scala.util.control.NonFatal. Otherwise, when trying to catch a StackOverflowError,
+    // we may try to load it but fail with another StackOverflowError and lose the original exception,
+    // see <https://groups.google.com/forum/#!topic/scala-user/kte6nak-zPM>.
+    val _ = NonFatal
+
+    assert(!args.contains("-d"))
+
+    val outPath = Paths.get("out")
+    Directory(outPath).createDirectory()
+
+    val tmpOut = Files.createTempDirectory(outPath.toAbsolutePath, "from-tasty-tmp")
+
+    val fromSourcesOut = Files.createDirectory(tmpOut.resolve("from-source"))
+
+    println("Compiling from .scala sources")
+    val compilation1 = dotc.Main.process("-d" +: fromSourcesOut.toString +: args)
+
+    if (compilation1.hasErrors) {
+      println("Failed compilation from sources")
+      Directory(tmpOut).deleteRecursively()
+      sys.exit(1)
+    }
+
+    val fromTastyOut = Files.createDirectory(tmpOut.resolve("from-tasty"))
+
+    val ext = "hasTasty"
+    val classes = Directory(fromSourcesOut).walk.filter(x => x.isFile && x.extension == ext).map { x =>
+      val source = x.toString
+      // transform foo/bar/Baz.hasTasty into foo.bar.Baz
+      source.substring(fromSourcesOut.toString.length + 1, source.length - ext.length - 1).replace('/', '.')
+    }.toList
+
+    val fromTastyArgs = {
+      "-from-tasty" ::
+      "-d" :: fromTastyOut.toString ::
+      insertClasspathInArgs(args.filterNot(_.endsWith(".scala")).toList, fromSourcesOut.toString) :::
+      classes
+    }
+
+    println("Compiling TASTY from .class sources")
+    val compilation2 = dotc.Main.process(fromTastyArgs.toArray)
+
+    if (compilation2.hasErrors) {
+      println("Failed compilation from TASTY")
+      println("Compilation input: " + fromSourcesOut)
+      // In this case we do not delete the generated class files to allow further debugging.
+      // For example `dotc -decompile` on one of the intermediate class files.
+      sys.exit(1)
+    }
+
+    Directory(tmpOut).deleteRecursively()
+  }
+
+  private def insertClasspathInArgs(args: List[String], cp: String): List[String] = {
+    val (beforeCp, fromCp) = args.span(_ != "-classpath")
+    val classpath = fromCp.drop(1).headOption.fold(cp)(_ + ":" + cp)
+    "-classpath" :: classpath :: beforeCp ::: fromCp.drop(2)
+  }
+}

project/Build.scala

@@ -630,11 +630,13 @@ object Build {
     val dottyLib = packageAll.value("dotty-library")
     val args0: List[String] = spaceDelimited("<arg>").parsed.toList
     val decompile = args0.contains("-decompile")
-    val args = args0.filter(arg => arg != "-repl" || arg != "-decompile")
+    val debugFromTasty = args0.contains("-Ythrough-tasty")
+    val args = args0.filter(arg => arg != "-repl" && arg != "-decompile" && arg != "-Ythrough-tasty")
 
     val main =
       if (repl) "dotty.tools.repl.Main"
       else if (decompile) "dotty.tools.dotc.decompiler.Main"
+      else if (debugFromTasty) "dotty.tools.dotc.fromtasty.Debug"
       else "dotty.tools.dotc.Main"
 
     val extraClasspath =