Allow users to pass a custom output dir on the command-line

integration/feature/output-directory/resources/build.mill

@@ -0,0 +1,8 @@
+package build
+
+import mill._
+import mill.scalalib._
+
+object `package` extends RootModule with ScalaModule {
+  def scalaVersion = scala.util.Properties.versionNumberString
+}

integration/feature/output-directory/src/OutputDirectoryTests.scala

@@ -0,0 +1,51 @@
+package mill.integration
+
+import mill.main.client.OutFiles
+import mill.testkit.UtestIntegrationTestSuite
+import utest._
+
+object OutputDirectoryTests extends UtestIntegrationTestSuite {
+
+  def tests: Tests = Tests {
+    test("Output directory sanity check") - integrationTest { tester =>
+      import tester._
+      eval("__.compile").isSuccess ==> true
+      val defaultOutDir = workspacePath / OutFiles.defaultOut
+      assert(os.isDir(defaultOutDir))
+    }
+
+    test("Output directory elsewhere in workspace") - integrationTest { tester =>
+      import tester._
+      eval(
+        "__.compile",
+        env = millTestSuiteEnv + ("MILL_OUTPUT" -> "testing/test-out")
+      ).isSuccess ==> true
+      val expectedOutDir = workspacePath / "testing/test-out"
+      val defaultOutDir = workspacePath / OutFiles.defaultOut
+      assert(os.isDir(expectedOutDir))
+      assert(!os.exists(defaultOutDir))
+    }
+
+    test("Output directory elsewhere in workspace via CLI") - integrationTest { tester =>
+      import tester._
+      eval(("--output", "testing/test-out", "__.compile")).isSuccess ==> true
+      val expectedOutDir = workspacePath / "testing/test-out"
+      val defaultOutDir = workspacePath / OutFiles.defaultOut
+      assert(os.isDir(expectedOutDir))
+      // Seems this one is created nonetheless, but is empty
+      assert(!os.exists(defaultOutDir) || os.list(defaultOutDir).isEmpty)
+    }
+
+    test("Output directory outside workspace") - integrationTest { tester =>
+      import tester._
+      val outDir = os.temp.dir() / "tmp-out"
+      eval(
+        "__.compile",
+        env = millTestSuiteEnv + ("MILL_OUTPUT" -> outDir.toString)
+      ).isSuccess ==> true
+      val defaultOutDir = workspacePath / OutFiles.defaultOut
+      assert(os.isDir(outDir))
+      assert(!os.exists(defaultOutDir))
+    }
+  }
+}

main/client/src/mill/main/client/OutFiles.java

@@ -5,10 +5,15 @@
  * and documentation about what they do
  */
 public class OutFiles {
+
+    final private static String envOutOrNull = System.getenv("MILL_OUTPUT");
+
+    final public static String defaultOut = "out";
+
     /**
      * Path of the Mill `out/` folder
      */
-    final public static String out = "out";
+    final public static String out = envOutOrNull == null ? defaultOut : envOutOrNull;
 
     /**
      * Path of the Mill "meta-build", used to compile the `build.sc` file so we can

runner/src/mill/runner/CodeGen.scala

@@ -15,7 +15,8 @@ object CodeGen {
       allScriptCode: Map[os.Path, String],
       targetDest: os.Path,
       enclosingClasspath: Seq[os.Path],
-      millTopLevelProjectRoot: os.Path
+      millTopLevelProjectRoot: os.Path,
+      output: os.Path
   ): Unit = {
     for (scriptSource <- scriptSources) {
       val scriptPath = scriptSource.path
@@ -94,6 +95,7 @@ object CodeGen {
             projectRoot,
             enclosingClasspath,
             millTopLevelProjectRoot,
+            output,
             scriptPath,
             scriptFolderPath,
             childAliases,
@@ -112,6 +114,7 @@ object CodeGen {
       projectRoot: os.Path,
       enclosingClasspath: Seq[os.Path],
       millTopLevelProjectRoot: os.Path,
+      output: os.Path,
       scriptPath: os.Path,
       scriptFolderPath: os.Path,
       childAliases: String,
@@ -126,7 +129,8 @@ object CodeGen {
       segments,
       scriptFolderPath,
       enclosingClasspath,
-      millTopLevelProjectRoot
+      millTopLevelProjectRoot,
+      output
     )
 
     val instrument = new ObjectDataInstrument(scriptCode)
@@ -182,13 +186,15 @@ object CodeGen {
       segments: Seq[String],
       scriptFolderPath: os.Path,
       enclosingClasspath: Seq[os.Path],
-      millTopLevelProjectRoot: os.Path
+      millTopLevelProjectRoot: os.Path,
+      output: os.Path
   ): String = {
     s"""import _root_.mill.runner.MillBuildRootModule
        |@_root_.scala.annotation.nowarn
        |object MillMiscInfo extends MillBuildRootModule.MillMiscInfo(
        |  ${enclosingClasspath.map(p => literalize(p.toString))},
        |  ${literalize(scriptFolderPath.toString)},
+       |  ${literalize(output.toString)},
        |  ${literalize(millTopLevelProjectRoot.toString)},
        |  _root_.scala.Seq(${segments.map(pprint.Util.literalize(_)).mkString(", ")})
        |)

runner/src/mill/runner/FileImportGraph.scala

@@ -43,7 +43,11 @@ object FileImportGraph {
    * starting from `build.mill`, collecting the information necessary to
    * instantiate the [[MillRootModule]]
    */
-  def parseBuildFiles(topLevelProjectRoot: os.Path, projectRoot: os.Path): FileImportGraph = {
+  def parseBuildFiles(
+      topLevelProjectRoot: os.Path,
+      projectRoot: os.Path,
+      output: os.Path
+  ): FileImportGraph = {
     val seenScripts = mutable.Map.empty[os.Path, String]
     val seenIvy = mutable.Set.empty[String]
     val seenRepo = mutable.ListBuffer.empty[(String, os.Path)]
@@ -193,7 +197,7 @@ object FileImportGraph {
             projectRoot,
             followLinks = true,
             skip = p =>
-              p == projectRoot / out ||
+              p == output ||
                 p == projectRoot / millBuild ||
                 (os.isDir(p) && !os.exists(p / nestedBuildFileName))
           )

runner/src/mill/runner/MillBuildBootstrap.scala

@@ -8,7 +8,7 @@ import mill.eval.Evaluator
 import mill.main.RunScript
 import mill.resolve.SelectMode
 import mill.define.{BaseModule, Discover, Segments}
-import mill.main.client.OutFiles._
+import mill.main.client.OutFiles.{millBuild, millRunnerState}
 
 import java.net.URLClassLoader
 
@@ -30,6 +30,7 @@ import java.net.URLClassLoader
 @internal
 class MillBuildBootstrap(
     projectRoot: os.Path,
+    output: os.Path,
     home: os.Path,
     keepGoing: Boolean,
     imports: Seq[String],
@@ -46,15 +47,15 @@ class MillBuildBootstrap(
 ) {
   import MillBuildBootstrap._
 
-  val millBootClasspath: Seq[os.Path] = prepareMillBootClasspath(projectRoot / out)
+  val millBootClasspath: Seq[os.Path] = prepareMillBootClasspath(output)
   val millBootClasspathPathRefs: Seq[PathRef] = millBootClasspath.map(PathRef(_, quick = true))
 
   def evaluate(): Watching.Result[RunnerState] = CliImports.withValue(imports) {
     val runnerState = evaluateRec(0)
 
     for ((frame, depth) <- runnerState.frames.zipWithIndex) {
       os.write.over(
-        recOut(projectRoot, depth) / millRunnerState,
+        recOut(output, depth) / millRunnerState,
         upickle.default.write(frame.loggedData, indent = 4),
         createFolders = true
       )
@@ -102,7 +103,8 @@ class MillBuildBootstrap(
       } else {
         val parsedScriptFiles = FileImportGraph.parseBuildFiles(
           projectRoot,
-          recRoot(projectRoot, depth) / os.up
+          recRoot(projectRoot, depth) / os.up,
+          output
         )
 
         if (parsedScriptFiles.millImport) evaluateRec(depth + 1)
@@ -111,6 +113,7 @@ class MillBuildBootstrap(
             new MillBuildRootModule.BootstrapModule(
               projectRoot,
               recRoot(projectRoot, depth),
+              output,
               millBootClasspath
             )(
               mill.main.RootModule.Info(
@@ -340,8 +343,8 @@ class MillBuildBootstrap(
     mill.eval.EvaluatorImpl(
       home,
       projectRoot,
-      recOut(projectRoot, depth),
-      recOut(projectRoot, depth),
+      recOut(output, depth),
+      recOut(output, depth),
       rootModule,
       PrefixLogger(logger, "", tickerContext = bootLogPrefix),
       classLoaderSigHash = millClassloaderSigHash,
@@ -422,8 +425,8 @@ object MillBuildBootstrap {
     projectRoot / Seq.fill(depth)(millBuild)
   }
 
-  def recOut(projectRoot: os.Path, depth: Int): os.Path = {
-    projectRoot / out / Seq.fill(depth)(millBuild)
+  def recOut(output: os.Path, depth: Int): os.Path = {
+    output / Seq.fill(depth)(millBuild)
   }
 
 }

runner/src/mill/runner/MillBuildRootModule.scala

@@ -121,7 +121,8 @@ abstract class MillBuildRootModule()(implicit
         parsed.seenScripts,
         T.dest,
         millBuildRootModuleInfo.enclosingClasspath,
-        millBuildRootModuleInfo.topLevelProjectRoot
+        millBuildRootModuleInfo.topLevelProjectRoot,
+        millBuildRootModuleInfo.output
       )
       Result.Success(Seq(PathRef(T.dest)))
     }
@@ -265,12 +266,14 @@ object MillBuildRootModule {
   class BootstrapModule(
       topLevelProjectRoot0: os.Path,
       projectRoot: os.Path,
+      output: os.Path,
       enclosingClasspath: Seq[os.Path]
   )(implicit baseModuleInfo: RootModule.Info) extends MillBuildRootModule()(
         implicitly,
         MillBuildRootModule.Info(
           enclosingClasspath,
           projectRoot,
+          output,
           topLevelProjectRoot0
         )
       ) {
@@ -281,25 +284,29 @@ object MillBuildRootModule {
   case class Info(
       enclosingClasspath: Seq[os.Path],
       projectRoot: os.Path,
+      output: os.Path,
       topLevelProjectRoot: os.Path
   )
 
   def parseBuildFiles(millBuildRootModuleInfo: MillBuildRootModule.Info): FileImportGraph = {
     FileImportGraph.parseBuildFiles(
       millBuildRootModuleInfo.topLevelProjectRoot,
-      millBuildRootModuleInfo.projectRoot / os.up
+      millBuildRootModuleInfo.projectRoot / os.up,
+      millBuildRootModuleInfo.output
     )
   }
 
   class MillMiscInfo(
       enclosingClasspath: Seq[String],
       projectRoot: String,
+      output: String,
       topLevelProjectRoot: String,
       segments: Seq[String]
   ) {
     implicit lazy val millBuildRootModuleInfo: MillBuildRootModule.Info = MillBuildRootModule.Info(
       enclosingClasspath.map(os.Path(_)),
       os.Path(projectRoot),
+      os.Path(output),
       os.Path(topLevelProjectRoot)
     )
     implicit lazy val millBaseModuleInfo: RootModule.Info = RootModule.Info(

runner/src/mill/runner/MillCliConfig.scala

@@ -1,6 +1,8 @@
 package mill.runner
 
 import mainargs.{Flag, Leftover, arg}
+import mill.api.WorkspaceRoot
+import mill.main.client.OutFiles
 
 case class MillCliConfig(
     @deprecated("No longer used", "Mill 0.12.0")
@@ -11,6 +13,12 @@ case class MillCliConfig(
         """(internal) The home directory where Mill looks for config and caches."""
     )
     home: os.Path = mill.api.Ctx.defaultHome,
+    @arg(
+      hidden = true,
+      doc =
+        """(internal) The directory to write Mill tasks output to."""
+    )
+    output: os.Path = os.Path(OutFiles.out, WorkspaceRoot.workspaceRoot),
     // We need to keep it, otherwise, a given --repl would be silently parsed as target and result in misleading error messages.
     // Instead we fail programmatically when this flag is set.
     @deprecated("No longer supported.", "Mill 0.11.0-M8")

runner/src/mill/runner/MillMain.scala

@@ -8,6 +8,7 @@ import mill.java9rtexport.Export
 import mill.api.{MillException, SystemStreams, WorkspaceRoot, internal}
 import mill.bsp.{BspContext, BspServerResult}
 import mill.main.BuildInfo
+import mill.main.client.OutFiles
 import mill.util.PrintLogger
 
 import java.lang.reflect.InvocationTargetException
@@ -230,6 +231,7 @@ object MillMain {
 
                       new MillBuildBootstrap(
                         projectRoot = WorkspaceRoot.workspaceRoot,
+                        output = config.output,
                         home = config.home,
                         keepGoing = config.keepGoing.value,
                         imports = config.imports,

testkit/src/mill/testkit/IntegrationTester.scala

@@ -91,7 +91,7 @@ object IntegrationTester {
       )
     }
 
-    private val millTestSuiteEnv = Map("MILL_TEST_SUITE" -> this.getClass().toString())
+    def millTestSuiteEnv = Map("MILL_TEST_SUITE" -> this.getClass().toString())
 
     /**
      * Helpers to read the `.json` metadata files belonging to a particular task

testkit/src/mill/testkit/IntegrationTesterBase.scala

@@ -34,21 +34,32 @@ trait IntegrationTesterBase {
     os.makeDir.all(workspacePath)
     Retry() {
       val tmp = os.temp.dir()
-      if (os.exists(workspacePath / out)) os.move.into(workspacePath / out, tmp)
+      if (os.exists(os.Path(out, workspacePath))) os.move.into(os.Path(out, workspacePath), tmp)
       os.remove.all(tmp)
     }
 
     os.list(workspacePath).foreach(os.remove.all(_))
-    os.list(workspaceSourcePath).filter(_.last != out).foreach(os.copy.into(_, workspacePath))
+    val outRelPathOpt = os.FilePath(out) match {
+      case relPath: os.RelPath if relPath.ups == 0 => Some(relPath)
+      case _ => None
+    }
+    os.list(workspaceSourcePath)
+      .filter(
+        outRelPathOpt match {
+          case None => _ => true
+          case Some(outRelPath) => !_.endsWith(outRelPath)
+        }
+      )
+      .foreach(os.copy.into(_, workspacePath))
   }
 
   /**
    * Remove any ID files to try and force them to exit
    */
   def removeServerIdFile(): Unit = {
-    if (os.exists(workspacePath / out)) {
+    if (os.exists(os.Path(out, workspacePath))) {
       val serverIdFiles = for {
-        outPath <- os.list.stream(workspacePath / out)
+        outPath <- os.list.stream(os.Path(out, workspacePath))
         if outPath.last.startsWith(millWorker)
       } yield outPath / serverId