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

bsp/worker/src/mill/bsp/worker/State.scala

@@ -39,7 +39,8 @@ private class State(projectRoot: os.Path, baseLogger: ColorLogger, debug: String
       threadCount = None,
       targetsAndParams = Seq("resolve", "_"),
       prevRunnerState = mill.runner.RunnerState.empty,
-      logger = baseLogger
+      logger = baseLogger,
+      needBuildSc = true
     ).evaluate()
 
     val rootModules0 = evaluated.result.frames

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
         }
@@ -58,12 +59,14 @@ object FileImportGraph {
 
           case Right(stmts) =>
             val fileImports = mutable.Set.empty[os.Path]
+            // we don't expect any new imports when using an empty dummy
+            if (useDummy) assert(fileImports.isEmpty)
             val transformedStmts = mutable.Buffer.empty[String]
             for ((stmt0, importTrees) <- Parsers.parseImportHooksWithIndices(stmts)) {
               walkStmt(s, stmt0, importTrees, fileImports, transformedStmts)
             }
             seenScripts(s) = transformedStmts.mkString
-            fileImports.foreach(walkScripts)
+            fileImports.foreach(walkScripts(_))
         }
       }
     }
@@ -116,7 +119,8 @@ object FileImportGraph {
       transformedStmts.append(stmt)
     }
 
-    walkScripts(projectRoot / "build.sc")
+    val useDummy = !os.exists(projectRoot / "build.sc")
+    walkScripts(projectRoot / "build.sc", useDummy)
     new FileImportGraph(
       seenScripts.toMap,
       seenRepo.toSeq,

runner/src/mill/runner/MillBuildBootstrap.scala

@@ -35,7 +35,8 @@ class MillBuildBootstrap(
     threadCount: Option[Int],
     targetsAndParams: Seq[String],
     prevRunnerState: RunnerState,
-    logger: ColorLogger
+    logger: ColorLogger,
+    needBuildSc: Boolean
 ) {
   import MillBuildBootstrap._
 
@@ -66,10 +67,23 @@ class MillBuildBootstrap(
 
     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.
+        lazy 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))
+          if (needBuildSc) {
+            RunnerState(None, Nil, Some(msg))
+          } else {
+            state match {
+              case RunnerState(bootstrapModuleOpt, frames, Some(error)) =>
+                // Add a potential clue (missing build.sc) to the underlying error message
+                RunnerState(bootstrapModuleOpt, frames, Some(msg + "\n" + error))
+              case state => state
+            }
+          }
         }
       } else {
         val parsedScriptFiles = FileImportGraph.parseBuildFiles(

runner/src/mill/runner/MillMain.scala

@@ -202,7 +202,8 @@ object MillMain {
                       threadCount = threadCount,
                       targetsAndParams = targetsAndParams,
                       prevRunnerState = prevState.getOrElse(stateCache),
-                      logger = logger
+                      logger = logger,
+                      needBuildSc = needBuildSc(config)
                     ).evaluate()
                   }
                 )
@@ -262,6 +263,25 @@ object MillMain {
     logger
   }
 
+  /**
+   * Determine, whether we need a `build.sc` or not.
+   */
+  private def needBuildSc(config: MillCliConfig): Boolean = {
+    // Tasks, for which running Mill without an existing buildfile is allowed.
+    val noBuildFileTaskWhitelist = Seq(
+      "init",
+      "version",
+      "mill.scalalib.giter8.Giter8Module/init"
+    )
+    val targetsAndParams = config.leftoverArgs.value
+    val whitelistMatch =
+      targetsAndParams.nonEmpty && noBuildFileTaskWhitelist.exists(targetsAndParams.head == _)
+    // Has the user additional/extra imports
+    // (which could provide additional commands that could make sense without a build.sc)
+    val extraPlugins = config.imports.nonEmpty
+    !(whitelistMatch || extraPlugins)
+  }
+
   def checkMillVersionFromFile(projectDir: os.Path, stderr: PrintStream) = {
     Seq(
       projectDir / ".config" / "mill-version",

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)
   }
 }