Fix init command and support running Mill without existing build.sc

I first added an integration test for the init command to reproduce the issues and make sure the feature now works.

Instead of failing in case there is no `build.sc`, we continue with an empty setup.

Only when we discover an error afterwards, we add a message about the missing `build.sc` to the underlying error message.

integration/feature/init/test/src/MillInitTests.scala

@@ -0,0 +1,16 @@
+package mill.integration
+package local
+import mill.bsp.Constants
+import utest._
+
+object MillInitTests extends IntegrationTestSuite {
+
+ def tests: Tests = Tests {
+ test("Mill init works") {
+ val workspacePath = initWorkspace()
+ eval("init", "com-lihaoyi/mill-scala-hello.g8", "--name=example") ==> true
+ val projFile = workspacePath / "example" / "build.sc"
+ assert(os.exists(projFile))
+ }
+ }
+}

runner/src/mill/runner/FileImportGraph.scala

@@ -20,7 +20,7 @@ case class FileImportGraph(
  */
 @internal
 object FileImportGraph {
- def backtickWrap(s: String) = if (encode(s) == s) s else "`" + s + "`"
+ def backtickWrap(s: String): String = if (encode(s) == s) s else "`" + s + "`"
 
  import mill.api.JsonFormatters.pathReadWrite
  implicit val readWriter: upickle.default.ReadWriter[FileImportGraph] = upickle.default.macroRW
@@ -38,13 +38,14 @@ object FileImportGraph {
  val errors = mutable.Buffer.empty[String]
  var millImport = false
 
- def walkScripts(s: os.Path): Unit = {
+ def walkScripts(s: os.Path, useDummy: Boolean = false): Unit = {
  importGraphEdges(s) = Nil
 
  if (!seenScripts.contains(s)) {
- val readFileEither = scala.util.Try(
- Parsers.splitScript(os.read(s), s.relativeTo(topLevelProjectRoot).toString)
- ) match {
+ val readFileEither = scala.util.Try {
+ val content = if (useDummy) "" else os.read(s)
+ Parsers.splitScript(content, s.relativeTo(topLevelProjectRoot).toString)
+ } match {
  case scala.util.Failure(ex) => Left(ex.getClass.getName + " " + ex.getMessage)
  case scala.util.Success(value) => value
  }
@@ -63,7 +64,7 @@ object FileImportGraph {
  walkStmt(s, stmt0, importTrees, fileImports, transformedStmts)
  }
  seenScripts(s) = transformedStmts.mkString
- fileImports.foreach(walkScripts)
+ fileImports.foreach(walkScripts(_))
  }
  }
  }
@@ -116,7 +117,7 @@ object FileImportGraph {
  transformedStmts.append(stmt)
  }
 
- walkScripts(projectRoot / "build.sc")
+ walkScripts(projectRoot / "build.sc", !os.exists(projectRoot / "build.sc"))
  new FileImportGraph(
  seenScripts.toMap,
  seenRepo.toSeq,

runner/src/mill/runner/MillBuildBootstrap.scala

@@ -60,16 +60,24 @@ class MillBuildBootstrap(
  }
 
  def evaluateRec(depth: Int): RunnerState = {
- // println(s"+evaluateRec($depth) " + recRoot(projectRoot, depth))
+ println(s"+evaluateRec($depth) " + recRoot(projectRoot, depth))
  val prevFrameOpt = prevRunnerState.frames.lift(depth)
  val prevOuterFrameOpt = prevRunnerState.frames.lift(depth - 1)
 
  val nestedState =
  if (depth == 0) {
- if (os.exists(recRoot(projectRoot, depth) / "build.sc")) evaluateRec(depth + 1)
+ // On this level we typically want assume a Mill project, which means we want to require an existing `build.sc`.
+ // Unfortunately, some targets also make sense without a `build.sc`, e.g. the `init` command.
+ // Hence we only report a missing `build.sc` as an problem if the command itself does not succeed.
+ val state = evaluateRec(depth + 1)
+ if (os.exists(recRoot(projectRoot, depth) / "build.sc")) state
  else {
- val msg = "build.sc file not found. Are you in a Mill project folder?"
- RunnerState(None, Nil, Some(msg))
+ state match {
+ case RunnerState(bootstrapModuleOpt, frames, Some(error)) =>
+ val msg = "build.sc file not found. Are you in a Mill project folder?\n"
+ RunnerState(bootstrapModuleOpt, frames, Some(msg + error))
+ case state => state
+ }
  }
  } else {
  val parsedScriptFiles = FileImportGraph.parseBuildFiles(

runner/src/mill/runner/RunnerState.scala

@@ -33,7 +33,10 @@ case class RunnerState(
  frames: Seq[RunnerState.Frame],
  errorOpt: Option[String]
 ) {
- def add(frame: RunnerState.Frame = RunnerState.Frame.empty, errorOpt: Option[String] = None) = {
+ def add(
+ frame: RunnerState.Frame = RunnerState.Frame.empty,
+ errorOpt: Option[String] = None
+ ): RunnerState = {
  this.copy(frames = Seq(frame) ++ frames, errorOpt = errorOpt)
  }
 }