Default exported Mill launchers to the main branch & add the --use-latest-mill-launchers toggle

Author: Gedochao

modules/cli/src/main/scala/scala/cli/commands/export0/Export.scala

@@ -99,11 +99,13 @@ object Export extends ScalaCommand[ExportOptions] {
     cache: FileCache[Task],
     projectName: Option[String],
     millVersion: String,
+    useLatestLaunchers: Boolean = true,
     logger: Logger
   ): MillProjectDescriptor = {
+    val launcherTag       = if useLatestLaunchers then "main" else millVersion
     val launcherArtifacts = Seq(
-      os.rel / "mill"     -> s"https://github.com/com-lihaoyi/mill/raw/$millVersion/mill",
-      os.rel / "mill.bat" -> s"https://github.com/com-lihaoyi/mill/raw/$millVersion/mill.bat"
+      os.rel / "mill"     -> s"https://github.com/com-lihaoyi/mill/raw/$launcherTag/mill",
+      os.rel / "mill.bat" -> s"https://github.com/com-lihaoyi/mill/raw/$launcherTag/mill.bat"
     )
     val launcherTasks = launcherArtifacts.map {
       case (path, url) =>
@@ -274,6 +276,7 @@ object Export extends ScalaCommand[ExportOptions] {
             cache = options.shared.coursierCache,
             projectName = options.project,
             millVersion = options.millVersion.getOrElse(Constants.millVersion),
+            useLatestLaunchers = options.useLatestMillLaunchers.getOrElse(true),
             logger = logger
           )
         else if shouldExportToMaven then

modules/cli/src/main/scala/scala/cli/commands/export0/ExportOptions.scala

@@ -62,6 +62,12 @@ final case class ExportOptions(
   )
   millVersion: Option[String] = None,
   @Group(HelpGroup.BuildToolExport.toString)
+  @Tag(tags.restricted)
+  @HelpMessage(
+    s"If set, latest Mill launchers from the main branch will be used, rather than the ones for the used version tag (true by default)"
+  )
+  useLatestMillLaunchers: Option[Boolean] = None,
+  @Group(HelpGroup.BuildToolExport.toString)
   @Tag(tags.experimental)
   @HelpMessage(
     s"Version of Maven Compiler Plugin to be used for the export (${Constants.defaultMavenVersion} by default)"

modules/integration/src/test/scala/scala/cli/integration/ExportMillTestDefinitions.scala

@@ -5,6 +5,8 @@ import os.RelPath
 
 import java.nio.charset.Charset
 
+import scala.util.Properties
+
 abstract class ExportMillTestDefinitions extends ScalaCliSuite
     with TestScalaVersionArgs
     with ExportCommonTestDefinitions
@@ -104,6 +106,16 @@ abstract class ExportMillTestDefinitions extends ScalaCliSuite
         jvmTestScalacOptions(className = "Hello", exportArgs = defaultExportCommandArgs)
       }
     }
+    if (!Properties.isMac || TestUtil.isM1) && !Properties.isWin then
+      // TODO enable this for intel Macs when Mill is bumped to 1.1.0 stable or newer: https://github.com/com-lihaoyi/mill/issues/6632
+      test(s"JVM scalac options with disabled latest launchers$commonTestDescriptionSuffix") {
+        TestUtil.retryOnCi() {
+          jvmTestScalacOptions(
+            className = "Hello",
+            exportArgs = defaultExportCommandArgs ++ Seq("--use-latest-mill-launchers=false")
+          )
+        }
+      }
     if !actualScalaVersion.startsWith("2.12") then
       test(s"JVM with a compiler plugin$commonTestDescriptionSuffix") {
         TestUtil.retryOnCi() {

website/docs/reference/cli-options.md

@@ -390,6 +390,10 @@ Version of SBT to be used for the export (1.11.7 by default)
 
 Version of Mill to be used for the export (1.0.6 by default)
 
+### `--use-latest-mill-launchers`
+
+If set, latest Mill launchers from the main branch will be used, rather than the ones for the used version tag (true by default)
+
 ### `--mvn-version`
 
 Version of Maven Compiler Plugin to be used for the export (3.8.1 by default)