Create empty class to enforce resolving ivy deps by mill for dummy modules

[lwronski]

When IntelliJ import Scala CLI BSP project, it throws:

Result: null
Error: {
  "code": -32603,
  "message": "Internal error.",
  "data": "java.util.concurrent.CompletionException: java.lang.Exception: Failure during task evaluation: dummy.amm[2.13.4].bspCompileClassesPath java.lang.NullPointerException: Cannot invoke \"mill.scalalib.api.CompilationResult.classes()\" because the return value of \"scala.collection.immutable.Seq.apply(int)\" is null\n    mill.scalalib.ScalaModule.$anonfun$bspCompileClassesPath$4(ScalaModule.scala:232)\n    mill.define.Task$TraverseCtx.evaluate(Task.scala:380)\n\tat java.base/java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:332)\n\tat java.base/java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:347)\n\tat java.base/java.util.concurrent.CompletableFuture$UniAccept.tryFire(CompletableFuture.java:708)\n\tat java.base/java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:510)\n\tat java.base/java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2162)\n\tat mill.bsp.MillBuildServer.$anonfun$completable$1(MillBuildServer.scala:722)\n\tat mill.bsp.MillBuildServer.$anonfun$completable$1$adapted(MillBuildServer.scala:710)\n\tat scala.concurrent.impl.Promise$Transformation.run(Promise.scala:484)\n\tat java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1395)\n\tat java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)\n\tat java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)\n\tat java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)\n\tat java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)\n\tat java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)\nCaused by: java.lang.Exception: Failure during task evaluation: dummy.amm[2.13.4].bspCompileClassesPath java.lang.NullPointerException: Cannot invoke \"mill.scalalib.api.CompilationResult.classes()\" because the return value of \"scala.collection.immutable.Seq.apply(int)\" is null\n    mill.scalalib.ScalaModule.$anonfun$bspCompileClassesPath$4(ScalaModule.scala:232)\n    mill.define.Task$TraverseCtx.evaluate(Task.scala:380)\n\tat mill.eval.Evaluator$.$anonfun$evalOrThrow$default$2$1(Evaluator.scala:1001)\n\tat mill.eval.Evaluator$EvalOrThrow.apply(Evaluator.scala:990)\n\tat mill.bsp.MillBuildServer.targetTasks(MillBuildServer.scala:683)\n\tat mill.bsp.MillScalaBuildServer.$anonfun$buildTargetScalacOptions$1(MillScalaBuildServer.scala:35)\n\tat mill.bsp.MillBuildServer.$anonfun$completable$1(MillBuildServer.scala:713)\n\t... 8 more\n"
}

To fix it, scala-cli creates dummy class to enforce compiling module by mill

[romanowski]

@lwronski shouldn't we report a bug for Scala Plugin? It shouldn't be needed

[lwronski]

I think the root of the problem is that scala-cli returned null for compile method in mill module. IMO it is our bug, not Scala Plugin.

def compile = T {
       resolvedRunIvyDeps()
       null: mill.scalalib.api.CompilationResult
     }