Change the scala compiler version, not scala library version.

Author: arturaz

runner/launcher/src/mill/launcher/CoursierClient.scala

@@ -15,10 +15,10 @@ object CoursierClient {
   /**
    * Resolves the classpath for the mill daemon.
    *
-   * @param scalaLibraryVersion the version of the scala library to use. If not specified, the version from the
-   *                            mill build is used.
+   * @param scalaVersion the version of the Scala to use. If not specified, the version that mill uses itself will be
+   *                     used.
    */
-  def resolveMillDaemon(scalaLibraryVersion: Option[String]): Array[String] = {
+  def resolveMillDaemon(scalaVersion: Option[String]): Array[String] = {
     val repositories = Await.result(Resolve().finalRepositories.future(), Duration.Inf)
     val coursierCache0 = FileCache[Task]()
       .withLogger(coursier.cache.loggers.RefreshLogger.create())
@@ -31,9 +31,9 @@ object CoursierClient {
             Module(Organization("com.lihaoyi"), ModuleName("mill-runner-daemon_3"), attributes = Map.empty),
             VersionConstraint(mill.client.BuildInfo.millVersion)
           )
-        ) ++ scalaLibraryVersion.map { version =>
+        ) ++ scalaVersion.map { version =>
           Dependency(
-            Module(Organization("org.scala-lang"), ModuleName("scala3-library_3"), attributes = Map.empty),
+            Module(Organization("org.scala-lang"), ModuleName("scala3-compiler_3"), attributes = Map.empty),
             VersionConstraint(version)
           )
         })

runner/launcher/src/mill/launcher/MillProcessLauncher.java

@@ -156,8 +156,8 @@ static String millJvmVersion() throws Exception {
     return loadMillConfigSingleValue("mill-jvm-version");
   }
 
-  static String millScalaLibraryVersion() throws Exception {
-    return loadMillConfigSingleValue("mill-scala-library-version");
+  static String millScalaVersion() throws Exception {
+    return loadMillConfigSingleValue("mill-scala-version");
   }
 
   static String loadMillConfigSingleValue(String key) throws Exception {
@@ -237,15 +237,15 @@ static List<String> millLaunchJvmCommand() throws Exception {
     // extra opts
     vmOptions.addAll(millJvmOpts());
 
-    var maybeScalaLibraryVersion = millScalaLibraryVersion();
+    var maybeScalaVersion = millScalaVersion();
     vmOptions.add("-XX:+HeapDumpOnOutOfMemoryError");
     vmOptions.add("-cp");
     var classPathCacheKey = 
       "mill:" + BuildInfo.millVersion + 
-      ",scala-library-version:" + (maybeScalaLibraryVersion == null ? "default" : maybeScalaLibraryVersion);
+      ",scala:" + (maybeScalaVersion == null ? "default" : maybeScalaVersion);
     String[] runnerClasspath = cachedComputedValue0(
         "resolve-runner", classPathCacheKey,
-         () -> CoursierClient.resolveMillDaemon(Option$.MODULE$.apply(maybeScalaLibraryVersion)),
+         () -> CoursierClient.resolveMillDaemon(Option$.MODULE$.apply(maybeScalaVersion)),
       arr -> {
           for (String s : arr) {
             if (!Files.exists(Paths.get(s))) return false;