Add simple way to use Scala 2 deps in a Dotty project

This is similar to the withDottyCompat method in the sbt-dotty plugin.

scalalib/src/mill/scalalib/Dep.scala

@@ -5,6 +5,8 @@ import upickle.default.{macroRW, ReadWriter => RW}
 import CrossVersion._
 
 case class Dep(dep: coursier.Dependency, cross: CrossVersion, force: Boolean) {
+  import Dep.isDotty
+
   def artifactName(binaryVersion: String, fullVersion: String, platformSuffix: String) = {
     val suffix = cross.suffixString(binaryVersion, fullVersion, platformSuffix)
     dep.module.name + suffix
@@ -17,12 +19,41 @@ case class Dep(dep: coursier.Dependency, cross: CrossVersion, force: Boolean) {
   def toDependency(binaryVersion: String, fullVersion: String, platformSuffix: String) =
     dep.copy(module = dep.module.copy(name = artifactName(binaryVersion, fullVersion, platformSuffix)))
   def withConfiguration(configuration: String): Dep = copy(dep = dep.copy(configuration = configuration))
+
+  /**
+    * If scalaVersion is a Dotty version, replace the cross-version suffix
+    * by the Scala 2.x version that the Dotty version is retro-compatible with,
+    * otherwise do nothing.
+    *
+    * This setting is useful when your build contains dependencies that have only
+    * been published with Scala 2.x, if you have:
+    * {{{
+    * def ivyDeps = Agg(ivy"a::b:c")
+    * }}}
+    * you can replace it by:
+    * {{{
+    * def ivyDeps = Agg(ivy"a::b:c".withDottyCompat(scalaVersion()))
+    * }}}
+    * This will have no effect when compiling with Scala 2.x, but when compiling
+    * with Dotty this will change the cross-version to a Scala 2.x one. This
+    * works because Dotty is currently retro-compatible with Scala 2.x.
+    */
+  def withDottyCompat(scalaVersion: String): Dep =
+    cross match {
+      case cross: Binary if isDotty(scalaVersion) =>
+        copy(cross = Constant(value = "_2.12", platformed = cross.platformed))
+      case _ =>
+        this
+    }
 }
 
 object Dep {
 
   val DefaultConfiguration = "default(compile)"
 
+  def isDotty(scalaVersion: String) =
+    scalaVersion.startsWith("0.")
+
   implicit def parse(signature: String): Dep = {
     val parts = signature.split(';')
     val module = parts.head

scalalib/src/mill/scalalib/Lib.scala

@@ -10,6 +10,7 @@ import javax.tools.ToolProvider
 import ammonite.ops._
 import ammonite.util.Util
 import coursier.{Cache, Dependency, Fetch, Repository, Resolution}
+import Dep.isDotty
 import mill.Agg
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
@@ -67,9 +68,6 @@ object Lib{
     }
   }
 
-  def isDotty(scalaVersion: String) =
-    scalaVersion.startsWith("0.")
-
   def grepJar(classPath: Agg[Path], name: String, version: String) = {
     val mavenStylePath = s"$name-$version.jar"
     val ivyStylePath = s"$version/$name.jar"

scalalib/src/mill/scalalib/ScalaModule.scala

@@ -8,6 +8,7 @@ import mill.define.TaskModule
 import mill.eval.{PathRef, Result}
 import mill.modules.Jvm
 import mill.modules.Jvm.{createJar, subprocess}
+import Dep.isDotty
 import Lib._
 import mill.util.Loose.Agg
 import mill.util.DummyInputStream

scalalib/test/resources/hello-dotty/foo/src/Main.scala

@@ -1,3 +1,5 @@
+import cats._, cats.data._, cats.implicits._
+
 trait Context
 
 object Main {
@@ -7,6 +9,8 @@ object Main {
   }
 
   def main(args: Array[String]): Unit = {
+    val x = Applicative[List].pure(1)
+    assert(x == List(1))
     val value = foo(implicit x => x + 1)
     assert(value == 2)
   }

scalalib/test/src/mill/scalalib/HelloWorldTests.scala

@@ -237,6 +237,7 @@ object HelloWorldTests extends TestSuite {
   object HelloDotty extends HelloBase{
     object foo extends ScalaModule {
       def scalaVersion = "0.9.0-RC1"
+      def ivyDeps = Agg(ivy"org.typelevel::cats-core:1.2.0".withDottyCompat(scalaVersion()))
      }
   }
 

scalalib/worker/src/mill/scalalib/worker/ScalaWorker.scala

@@ -9,7 +9,8 @@ import mill.Agg
 import mill.eval.PathRef
 import mill.scalalib.{CompilationResult, Lib, TestRunner}
 import xsbti.compile.{CompilerCache => _, FileAnalysisStore => _, ScalaInstance => _, _}
-import mill.scalalib.Lib.{grepJar, isDotty, scalaBinaryVersion}
+import mill.scalalib.Dep.isDotty
+import mill.scalalib.Lib.{grepJar, scalaBinaryVersion}
 import mill.util.{Ctx, PrintLogger}
 import sbt.internal.inc._
 import sbt.internal.util.{ConsoleOut, MainAppender}