Add a warning message when launching from scala.

Author: bishabosha

compiler/src/dotty/tools/MainGenericRunner.scala

@@ -266,6 +266,22 @@ object MainGenericRunner {
           run(settings.withExecuteMode(ExecuteMode.Run))
         else
           run(settings.withExecuteMode(ExecuteMode.Repl))
+    end run
+
+    val ranByCoursierBootstrap =
+      sys.props.isDefinedAt("coursier.mainJar")
+      || sys.props.get("bootstrap.mainClass").filter(_ == "dotty.tools.MainGenericRunner").isDefined
+
+    val silenced = sys.props.get("scala.use_legacy_launcher") == Some("true")
+
+    if !silenced then
+      Console.err.println(s"[warning] MainGenericRunner class is deprecated since Scala 3.5.0, and Scala CLI features will not work.")
+      Console.err.println(s"[warning] Please be sure to update to the Scala CLI launcher to use the new features.")
+      if ranByCoursierBootstrap then
+        Console.err.println(s"[warning] It appears that your Coursier-based Scala installation is misconfigured.")
+        Console.err.println(s"[warning] To update to the new Scala CLI runner, please update (coursier, cs) commands first before re-installing scala.")
+      Console.err.println(s"[warning] Check the Scala 3.5.0 release notes to troubleshoot your installation.")
+
 
     run(settings) match
       case Some(ex: (StringDriverException | ScriptingException)) => errorFn(ex.getMessage)

compiler/test-coursier/dotty/tools/coursier/CoursierScalaTests.scala

@@ -166,7 +166,7 @@ object CoursierScalaTests:
       case Nil => args
       case _ => "--" +: args
     val newJOpts = jOpts.map(s => s"--java-opt ${s.stripPrefix("-J")}").mkString(" ")
-    execCmd("./cs", (s"""launch "org.scala-lang:scala3-compiler_3:${sys.env("DOTTY_BOOTSTRAPPED_VERSION")}" $newJOpts --main-class "$entry" --property "scala.usejavacp=true"""" +: newOptions)*)._2
+    execCmd("./cs", (s"""launch "org.scala-lang:scala3-compiler_3:${sys.env("DOTTY_BOOTSTRAPPED_VERSION")}" $newJOpts --main-class "$entry" --property "scala.usejavacp=true" --property "scala.use_legacy_launcher=true"""" +: newOptions)*)._2
 
   /** Get coursier script */
   @BeforeClass def setup(): Unit =