Make sbt aware of Dotty

This small set of changes, together with the compiler-bridge I wrote
(https://github.com/smarter/dotty-bridge) enables us to compile code
using Dotty in sbt, see https://github.com/smarter/dotty-example-project
for an example.

compile/src/main/scala/sbt/compiler/CompilerArguments.scala

@@ -30,7 +30,11 @@ final class CompilerArguments(scalaInstance: xsbti.compile.ScalaInstance, cp: xs
     }
   def finishClasspath(classpath: Seq[File]): Seq[File] =
     filterLibrary(classpath) ++ include(cp.compiler, scalaInstance.compilerJar) ++ include(cp.extra, scalaInstance.otherJars: _*)
-  private[this] def include(flag: Boolean, jars: File*) = if (flag) jars else Nil
+  private[this] def include(flag: Boolean, jars: File*) =
+    if (flag || ScalaInstance.isDotty(scalaInstance.version))
+      jars
+    else
+      Nil
   private[this] def abs(files: Seq[File]) = files.map(_.getAbsolutePath).sortWith(_ < _)
   private[this] def checkScalaHomeUnset(): Unit = {
     val scalaHome = System.getProperty("scala.home")

compile/src/main/scala/sbt/compiler/RawCompiler.scala

@@ -21,16 +21,29 @@ class RawCompiler(val scalaInstance: xsbti.compile.ScalaInstance, cp: ClasspathO
 
     val arguments = compilerArguments(sources, classpath, Some(outputDirectory), options)
     log.debug("Plain interface to Scala compiler " + scalaInstance.actualVersion + "  with arguments: " + arguments.mkString("\n\t", "\n\t", ""))
-    val mainClass = Class.forName("scala.tools.nsc.Main", true, scalaInstance.loader)
-    val process = mainClass.getMethod("process", classOf[Array[String]])
-    process.invoke(null, arguments.toArray)
-    checkForFailure(mainClass, arguments.toArray)
+    val args = arguments.toArray
+    val reporter =
+      if (ScalaInstance.isDotty(scalaInstance.version)) {
+        val mainClass = Class.forName("dotty.tools.dotc.Main", true, scalaInstance.loader)
+        val process = mainClass.getMethod("process", classOf[Array[String]])
+        process.invoke(null, args)
+      } else {
+        val mainClass = Class.forName("scala.tools.nsc.Main", true, scalaInstance.loader)
+        val process = mainClass.getMethod("process", classOf[Array[String]])
+        process.invoke(null, arguments.toArray)
+        mainClass.getMethod("reporter").invoke(null)
+      }
+    checkForFailure(reporter, arguments.toArray)
   }
   def compilerArguments = new CompilerArguments(scalaInstance, cp)
-  protected def checkForFailure(mainClass: Class[_], args: Array[String]): Unit = {
-    val reporter = mainClass.getMethod("reporter").invoke(null)
+  protected def checkForFailure(reporter: AnyRef, args: Array[String]): Unit = {
     val failed = reporter.getClass.getMethod("hasErrors").invoke(reporter).asInstanceOf[Boolean]
     if (failed) throw new CompileFailed(args, "Plain compile failed", Array())
   }
+  @deprecated("Use `checkForFailure(AnyRef, Array[String])`", "0.13.10")
+  protected def checkForFailure(mainClass: Class[_], args: Array[String]): Unit = {
+    val reporter = mainClass.getMethod("reporter").invoke(null)
+    checkForFailure(reporter, args)
+  }
 }
 class CompileFailed(val arguments: Array[String], override val toString: String, val problems: Array[xsbti.Problem]) extends xsbti.CompileFailed with FeedbackProvidedException

ivy/src/main/scala/sbt/IvyScala.scala

@@ -17,12 +17,20 @@ object ScalaArtifacts {
   val LibraryID = ScalaLibraryID
   val CompilerID = ScalaCompilerID
   val ReflectID = "scala-reflect"
+  val DottyIDPrefix = "dotty"
+
+  def dottyID(binaryVersion: String): String = s"${DottyIDPrefix}_${binaryVersion}"
+
   def libraryDependency(version: String): ModuleID = ModuleID(Organization, LibraryID, version)
 
-  private[sbt] def toolDependencies(org: String, version: String): Seq[ModuleID] = Seq(
-    scalaToolDependency(org, ScalaArtifacts.CompilerID, version),
-    scalaToolDependency(org, ScalaArtifacts.LibraryID, version)
-  )
+  private[sbt] def toolDependencies(org: String, version: String, isDotty: Boolean = false): Seq[ModuleID] =
+    if (isDotty)
+      Seq(ModuleID(org, DottyIDPrefix, version, Some(Configurations.ScalaTool.name + "->compile"),
+        crossVersion = CrossVersion.binary))
+    else
+      Seq(scalaToolDependency(org, ScalaArtifacts.CompilerID, version),
+        scalaToolDependency(org, ScalaArtifacts.LibraryID, version))
+
   private[this] def scalaToolDependency(org: String, id: String, version: String): ModuleID =
     ModuleID(org, id, version, Some(Configurations.ScalaTool.name + "->default,optional(default)"))
 }

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

@@ -390,7 +390,11 @@ object Defaults extends BuildCommon {
     def file(id: String) = files(id).headOption getOrElse sys.error(s"Missing ${id}.jar")
     val allFiles = toolReport.modules.flatMap(_.artifacts.map(_._2))
     val libraryJar = file(ScalaArtifacts.LibraryID)
-    val compilerJar = file(ScalaArtifacts.CompilerID)
+    val compilerJar =
+      if (ScalaInstance.isDotty(scalaVersion.value))
+        file(ScalaArtifacts.dottyID(scalaBinaryVersion.value))
+      else
+        file(ScalaArtifacts.CompilerID)
     val otherJars = allFiles.filterNot(x => x == libraryJar || x == compilerJar)
     ScalaInstance(scalaVersion.value, libraryJar, compilerJar, otherJars: _*)(makeClassLoader(state.value))
   }
@@ -1250,8 +1254,11 @@ object Classpaths {
       val pluginAdjust = if (sbtPlugin.value) sbtDependency.value.copy(configurations = Some(Provided.name)) +: base else base
       if (scalaHome.value.isDefined || ivyScala.value.isEmpty || !managedScalaInstance.value)
         pluginAdjust
-      else
-        ScalaArtifacts.toolDependencies(scalaOrganization.value, scalaVersion.value) ++ pluginAdjust
+      else {
+        val version = scalaVersion.value
+        val isDotty = ScalaInstance.isDotty(version)
+        ScalaArtifacts.toolDependencies(scalaOrganization.value, version, isDotty) ++ pluginAdjust
+      }
     }
   )
   @deprecated("Split into ivyBaseSettings and jvmBaseSettings.", "0.13.2")

notes/0.13.10/dotty-awareness.md

@@ -0,0 +1,16 @@
+
+  [Dotty]: https://github.com/lampepfl/dotty
+  [@smarter]: https://github.com/smarter
+
+### Fixes with compatibility implications
+
+### Improvements
+
+- sbt is now aware of [Dotty][Dotty], it will assume
+  that Dotty is used when `scalaVersion` starts with `0.`, the sbt
+  compiler-bridge does not support Dotty but a separate compiler-bridge is being
+  developed at https://github.com/smarter/dotty-bridge and an example project
+  that uses it is available at https://github.com/smarter/dotty-example-project
+  by [@smarter][@smarter].
+
+### Bug fixes

util/classpath/src/main/scala/sbt/ScalaInstance.scala

@@ -41,6 +41,11 @@ object ScalaInstance {
   val ScalaOrg = ScalaOrganization
   val VersionPrefix = "version "
 
+  def isDotty(version: String): Boolean =
+    // We rely on the fact that the first public version of Scala was 1.0 and
+    // that Dotty will keep being version 0.x for a long time.
+    version.startsWith("0.")
+
   def apply(org: String, version: String, launcher: xsbti.Launcher): ScalaInstance =
     // Due to incompatibility with previous launchers if scalaOrg has default value revert to an existing method
     if (org == ScalaOrg)