BSP: Fix mill-build module interpretation in IntelliJ IDEA (#1876)

IntelliJ IDEA does not understand single files as sources, which we would need to handle `build.sc`. But when we use the project root directory, it creates a problem where IntelliJ may interpret a `root.Foo` class in a sub-project `sub/foo` as `sub.foo.src.root.Foo` class.

Pull request: #1876

bsp/src/mill/bsp/MillBuildServer.scala

@@ -89,6 +89,7 @@ class MillBuildServer(
   var clientInitialized = false
   var shutdownRequested = false
   var clientWantsSemanticDb = false
+  var clientIsIntelliJ = false
 
   class State(val evaluator: Evaluator) {
     private[this] object internal {
@@ -221,6 +222,11 @@ class MillBuildServer(
       capabilities.setJvmRunEnvironmentProvider(true)
       capabilities.setJvmTestEnvironmentProvider(true)
 
+      request.getDisplayName match {
+        case "IntelliJ-BSP" => clientIsIntelliJ = true
+        case _ => clientIsIntelliJ = false
+      }
+
       request.getData match {
         case d: JsonObject =>
           log.debug(s"extra data: ${d} of type ${d.getClass}")
@@ -335,6 +341,15 @@ class MillBuildServer(
         targetIds = sourcesParams.getTargets.asScala.toSeq,
         agg = (items: Seq[SourcesItem]) => new SourcesResult(items.asJava)
       ) {
+        case (id, module: MillBuildTarget) if clientIsIntelliJ =>
+          T.task {
+            val sources = new SourcesItem(
+              id,
+              module.dummySources().map(p => sourceItem(p.path, true)).asJava
+            )
+            sources.setRoots(Seq(sanitizeUri(evaluator.rootModule.millSourcePath)).asJava)
+            sources
+          }
 //        case (id, `millBuildTarget`) =>
 //          T.task {
 //            new SourcesItem(

scalalib/src/mill/scalalib/bsp/BspModule.scala

@@ -142,4 +142,7 @@ trait MillBuildTarget // (rootModule: BaseModule, ctx0: mill.define.Ctx)
     os.makeDir(T.dest / "classes")
     CompilationResult(T.dest / "dummy", PathRef(T.dest / "classes"))
   }
+
+  /** Used in BSP IntelliJ, which can only work with directories */
+  def dummySources: Sources = T.sources(T.dest)
 }