Add -Xmixin-force-forwarders

This controls when mixin forwarders are generated like the homonymous
flag in scalac, for now we default to "junit" to keep our existing
behavior, next commit will change it to "true" to match scalac.

Author: smarter

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

@@ -1,6 +1,7 @@
 package dotty.tools.dotc
 package config
 
+import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.io.{ Directory, PlainDirectory, AbstractFile }
 import PathResolver.Defaults
 import rewrites.Rewrites
@@ -76,6 +77,18 @@ class ScalaSettings extends Settings.SettingGroup {
   val XfatalWarnings: Setting[Boolean] = BooleanSetting("-Xfatal-warnings", "Fail the compilation if there are any warnings.")
   val XverifySignatures: Setting[Boolean] = BooleanSetting("-Xverify-signatures", "Verify generic signatures in generated bytecode.")
 
+  val XmixinForceForwarders = ChoiceSetting(
+    name    = "-Xmixin-force-forwarders",
+    helpArg = "mode",
+    descr   = "Generate forwarder methods in classes inhering concrete methods from traits.",
+    choices = List("true", "junit", "false"),
+    default = "junit")
+
+  object mixinForwarderChoices {
+    def isTruthy(implicit ctx: Context) = XmixinForceForwarders.value == "true"
+    def isAtLeastJunit(implicit ctx: Context) = isTruthy || XmixinForceForwarders.value == "junit"
+  }
+
   /** -Y "Private" settings */
   val YoverrideVars: Setting[Boolean] = BooleanSetting("-Yoverride-vars", "Allow vars to be overridden.")
   val Yhelp: Setting[Boolean] = BooleanSetting("-Y", "Print a synopsis of private options.")

compiler/src/dotty/tools/dotc/transform/MixinOps.scala

@@ -59,12 +59,13 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
     def hasNonInterfaceDefinition = competingMethods.exists(!_.owner.is(Trait)) // there is a definition originating from class
     !meth.isConstructor &&
     meth.is(Method, butNot = PrivateOrAccessorOrDeferred) &&
-    (meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition || needsJUnit4Fix(meth) ) &&
+    (ctx.settings.mixinForwarderChoices.isTruthy || meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition || needsJUnit4Fix(meth)) &&
     isCurrent(meth)
   }
 
   private def needsJUnit4Fix(meth: Symbol): Boolean = {
-    meth.annotations.nonEmpty && JUnit4Annotations.exists(annot => meth.hasAnnotation(annot))
+    meth.annotations.nonEmpty && JUnit4Annotations.exists(annot => meth.hasAnnotation(annot)) &&
+      ctx.settings.mixinForwarderChoices.isAtLeastJunit
   }
 
   final val PrivateOrAccessor: FlagSet = Private | Accessor