Classify Zinc workers by zinc version

Each version of zinc comes with its own compiler bridges on every
version of Scala. Given that two different versions of Mill can use
two different versions of Zinc we must classify zinc workers by zinc
version (in addition to scala version).

build.sc

@@ -309,6 +309,25 @@ object scalalib extends MillModule {
     def testArgs = T{Seq(
       "-DMILL_SCALA_WORKER=" + runClasspath().map(_.path).mkString(",")
     )}
+
+    override def generatedSources = T{
+      val dest = T.ctx.dest
+      val artifacts = T.traverse(dev.moduleDeps)(_.publishSelfDependency)()
+      os.write(dest / "Versions.scala",
+        s"""package mill.scalalib.worker
+           |
+           |/**
+           | * Dependency versions.
+           | * Generated from mill in build.sc.
+           | */
+           |object Versions {
+           |  /** Version of Zinc. */
+           |  val zinc = "${Deps.zinc.dep.version}"
+           |}
+           |
+           |""".stripMargin)
+      super.generatedSources() ++ Seq(PathRef(dest))
+    }
   }
 }
 

scalalib/worker/src/ZincWorkerImpl.scala

@@ -181,7 +181,7 @@ class ZincWorkerImpl(compilerBridge: Either[
     compilerBridge match {
       case Right(compiled) => compiled(scalaVersion)
       case Left((ctx0, bridgeProvider)) =>
-        val workingDir = ctx0.dest / scalaVersion
+        val workingDir = ctx0.dest / s"zinc-${Versions.zinc}" / scalaVersion
         val lock = synchronized(compilerBridgeLocks.getOrElseUpdate(scalaVersion, new Object()))
         val compiledDest = workingDir / 'compiled
         lock.synchronized{