improvement: Detect objects with main class in scripts

modules/build/src/main/scala/scala/build/ScopedSources.scala

@@ -62,14 +62,14 @@ final case class ScopedSources(
   ): Either[BuildException, Sources] = either {
     val combinedOptions = combinedBuildOptions(scope, baseOptions)
 
-    val codeWrapper = ScriptPreprocessor.getScriptWrapper(combinedOptions)
+    val codeWrapper = ScriptPreprocessor.getScriptWrapper(combinedOptions, logger)
 
     val wrappedScripts = unwrappedScripts
       .flatMap(_.valueFor(scope).toSeq)
       .map(_.wrap(codeWrapper))
 
     codeWrapper match {
-      case _: AppCodeWrapper.type if wrappedScripts.size > 1 =>
+      case _: AppCodeWrapper if wrappedScripts.size > 1 =>
         wrappedScripts.find(_.originalPath.exists(_._1.toString == "main.sc"))
           .foreach(_ => logger.diagnostic(WarningMessages.mainScriptNameClashesWithAppWrapper))
       case _ => ()

modules/build/src/main/scala/scala/build/internal/AppCodeWrapper.scala

@@ -1,6 +1,6 @@
 package scala.build.internal
 
-case object AppCodeWrapper extends CodeWrapper {
+case class AppCodeWrapper(scalaVersion: String, log: String => Unit) extends CodeWrapper {
   override def mainClassObject(className: Name) = className
 
   def apply(
@@ -12,13 +12,19 @@ case object AppCodeWrapper extends CodeWrapper {
   ) = {
     val wrapperObjectName = indexedWrapperName.backticked
 
+    val mainObject = WrapperUtils.mainObjectInScript(scalaVersion, code)
+    val invokeMain = mainObject match
+      case WrapperUtils.ScriptMainMethod.Exists(name) => s"\n$name.main(args)"
+      case otherwise =>
+        otherwise.warningMessage.foreach(log)
+        ""
     val packageDirective =
       if (pkgName.isEmpty) "" else s"package ${AmmUtil.encodeScalaSourcePath(pkgName)}" + "\n"
     val top = AmmUtil.normalizeNewlines(
       s"""$packageDirective
          |
          |object $wrapperObjectName extends App {
-         |val scriptPath = \"\"\"$scriptPath\"\"\"
+         |val scriptPath = \"\"\"$scriptPath\"\"\"$invokeMain
          |""".stripMargin
     )
     val bottom = AmmUtil.normalizeNewlines(

modules/build/src/main/scala/scala/build/internal/ClassCodeWrapper.scala

@@ -5,7 +5,7 @@ package scala.build.internal
   * running interconnected scripts using Scala CLI <br> <br> Incompatible with Scala 2 - it uses
   * Scala 3 feature 'export'<br> Incompatible with native JS members - the wrapper is a class
   */
-case object ClassCodeWrapper extends CodeWrapper {
+case class ClassCodeWrapper(scalaVersion: String, log: String => Unit) extends CodeWrapper {
 
   override def mainClassObject(className: Name): Name =
     Name(className.raw ++ "_sc")
@@ -16,8 +16,16 @@ case object ClassCodeWrapper extends CodeWrapper {
     extraCode: String,
     scriptPath: String
   ) = {
+
+    val mainObject = WrapperUtils.mainObjectInScript(scalaVersion, code)
+    val mainInvocation = mainObject match
+      case WrapperUtils.ScriptMainMethod.Exists(name) => s"script.$name.main(args)"
+      case otherwise =>
+        otherwise.warningMessage.foreach(log)
+        s"val _ = script.hashCode()"
+
     val name             = mainClassObject(indexedWrapperName).backticked
-    val wrapperClassName = Name(indexedWrapperName.raw ++ "$_").backticked
+    val wrapperClassName = scala.build.internal.Name(indexedWrapperName.raw ++ "$_").backticked
     val mainObjectCode =
       AmmUtil.normalizeNewlines(s"""|object $name {
                                     |  private var args$$opt0 = Option.empty[Array[String]]
@@ -33,7 +41,7 @@ case object ClassCodeWrapper extends CodeWrapper {
                                     |
                                     |  def main(args: Array[String]): Unit = {
                                     |    args$$set(args)
-                                    |    val _ = script.hashCode() // hashCode to clear scalac warning about pure expression in statement position
+                                    |    $mainInvocation // hashCode to clear scalac warning about pure expression in statement position
                                     |  }
                                     |}
                                     |

modules/build/src/main/scala/scala/build/internal/ObjectCodeWrapper.scala

@@ -4,7 +4,7 @@ package scala.build.internal
   * or/and not using JS native prefer [[ClassCodeWrapper]], since it prevents deadlocks when running
   * threads from script
   */
-case object ObjectCodeWrapper extends CodeWrapper {
+case class ObjectCodeWrapper(scalaVersion: String, log: String => Unit) extends CodeWrapper {
 
   override def mainClassObject(className: Name): Name =
     Name(className.raw ++ "_sc")
@@ -15,12 +15,19 @@ case object ObjectCodeWrapper extends CodeWrapper {
     extraCode: String,
     scriptPath: String
   ) = {
+    val mainObject         = WrapperUtils.mainObjectInScript(scalaVersion, code)
     val name               = mainClassObject(indexedWrapperName).backticked
     val aliasedWrapperName = name + "$$alias"
-    val funHashCodeMethod =
+    val realScript =
       if (name == "main_sc")
-        s"$aliasedWrapperName.alias.hashCode()" // https://github.com/VirtusLab/scala-cli/issues/314
-      else s"${indexedWrapperName.backticked}.hashCode()"
+        s"$aliasedWrapperName.alias" // https://github.com/VirtusLab/scala-cli/issues/314
+      else s"${indexedWrapperName.backticked}"
+
+    val funHashCodeMethod = mainObject match
+      case WrapperUtils.ScriptMainMethod.Exists(name) => s"$realScript.$name.main(args)"
+      case otherwise =>
+        otherwise.warningMessage.foreach(log)
+        s"val _ = $realScript.hashCode()"
     // We need to call hashCode (or any other method so compiler does not report a warning)
     val mainObjectCode =
       AmmUtil.normalizeNewlines(s"""|object $name {
@@ -34,7 +41,7 @@ case object ObjectCodeWrapper extends CodeWrapper {
                                     |  }
                                     |  def main(args: Array[String]): Unit = {
                                     |    args$$set(args)
-                                    |    val _ = $funHashCodeMethod // hashCode to clear scalac warning about pure expression in statement position
+                                    |    $funHashCodeMethod // hashCode to clear scalac warning about pure expression in statement position
                                     |  }
                                     |}
                                     |""".stripMargin)

modules/build/src/main/scala/scala/build/internal/WrapperUtils.scala

@@ -0,0 +1,88 @@
+package scala.build.internal
+
+import scala.build.internal.util.WarningMessages
+
+object WrapperUtils {
+
+  enum ScriptMainMethod:
+    case Exists(name: String)
+    case Multiple(names: Seq[String])
+    case ToplevelStatsPresent
+    case ToplevelStatsWithMultiple(names: Seq[String])
+    case NoMain
+
+    def warningMessage: List[String] =
+      this match
+        case ScriptMainMethod.Multiple(names) =>
+          List(WarningMessages.multipleMainObjectsInScript(names))
+        case ScriptMainMethod.ToplevelStatsPresent => List(
+            WarningMessages.mixedToplvelAndObjectInScript
+          )
+        case ToplevelStatsWithMultiple(names) =>
+          List(
+            WarningMessages.multipleMainObjectsInScript(names),
+            WarningMessages.mixedToplvelAndObjectInScript
+          )
+        case _ => Nil
+
+  def mainObjectInScript(scalaVersion: String, code: String): ScriptMainMethod =
+    import scala.meta.*
+
+    val scriptDialect =
+      if scalaVersion.startsWith("3") then dialects.Scala3Future else dialects.Scala213Source3
+
+    given Dialect = scriptDialect.withAllowToplevelStatements(true).withAllowToplevelTerms(true)
+    val parsedCode = code.parse[Source] match
+      case Parsed.Success(Source(stats)) => stats
+      case _                             => Nil
+
+    // Check if there is a main function defined inside an object
+    def checkSignature(defn: Defn.Def) =
+      defn.paramClauseGroups match
+        case List(Member.ParamClauseGroup(
+              Type.ParamClause(Nil),
+              List(Term.ParamClause(
+                List(Term.Param(
+                  Nil,
+                  _: Term.Name,
+                  Some(Type.Apply.After_4_6_0(
+                    Type.Name("Array"),
+                    Type.ArgClause(List(Type.Name("String")))
+                  )),
+                  None
+                )),
+                None
+              ))
+            )) => true
+        case _ => false
+
+    def noToplevelStatements = parsedCode.forall {
+      case _: Term => false
+      case _       => true
+    }
+
+    def hasMainSignature(templ: Template) = templ.body.stats.exists {
+      case defn: Defn.Def =>
+        defn.name.value == "main" && checkSignature(defn)
+      case _ => false
+    }
+    def extendsApp(templ: Template) = templ.inits match
+      case Init.After_4_6_0(Type.Name("App"), _, Nil) :: Nil => true
+      case _                                                 => false
+    val potentialMains = parsedCode.collect {
+      case Defn.Object(_, objName, templ) if extendsApp(templ) || hasMainSignature(templ) =>
+        Seq(objName.value)
+    }.flatten
+
+    potentialMains match
+      case head :: Nil if noToplevelStatements =>
+        ScriptMainMethod.Exists(head)
+      case head :: Nil =>
+        ScriptMainMethod.ToplevelStatsPresent
+      case Nil => ScriptMainMethod.NoMain
+      case seq if noToplevelStatements =>
+        ScriptMainMethod.Multiple(seq)
+      case seq =>
+        ScriptMainMethod.ToplevelStatsWithMultiple(seq)
+
+}

modules/build/src/main/scala/scala/build/internal/util/WarningMessages.scala

@@ -105,6 +105,12 @@ object WarningMessages {
   val offlineModeBloopJvmNotFound =
     "Offline mode is ON and a JVM for Bloop could not be fetched from the local cache, using scalac as fallback"
 
+  def multipleMainObjectsInScript(names: Seq[String]) =
+    s"Only a single main is allowed within scripts. Multiple main classes were found in the script: ${names.mkString(", ")}"
+
+  def mixedToplvelAndObjectInScript =
+    "Script contains objects with main methods and top-level statements, only the latter will be run."
+
   def directivesInMultipleFilesWarning(
     projectFilePath: String,
     pathsToReport: Iterable[String] = Nil

modules/build/src/main/scala/scala/build/preprocessing/ScriptPreprocessor.scala

@@ -134,7 +134,7 @@ case object ScriptPreprocessor extends Preprocessor {
     (codeWrapper: CodeWrapper) =>
       if (containsMainAnnot) logger.diagnostic(
         codeWrapper match {
-          case _: AppCodeWrapper.type =>
+          case _: AppCodeWrapper =>
             WarningMessages.mainAnnotationNotSupported( /* annotationIgnored */ true)
           case _ => WarningMessages.mainAnnotationNotSupported( /* annotationIgnored */ false)
         }
@@ -157,24 +157,27 @@ case object ScriptPreprocessor extends Preprocessor {
     * @return
     *   code wrapper compatible with provided BuildOptions
     */
-  def getScriptWrapper(buildOptions: BuildOptions): CodeWrapper = {
+  def getScriptWrapper(buildOptions: BuildOptions, logger: Logger): CodeWrapper = {
     val effectiveScalaVersion =
       buildOptions.scalaOptions.scalaVersion.flatMap(_.versionOpt)
         .orElse(buildOptions.scalaOptions.defaultScalaVersion)
         .getOrElse(Constants.defaultScalaVersion)
+    def logWarning(msg: String) = logger.diagnostic(msg)
 
     def objectCodeWrapperForScalaVersion =
       // AppObjectWrapper only introduces the 'main.sc' restriction when used in Scala 3, there's no gain in using it with Scala 3
-      if effectiveScalaVersion.startsWith("2") then AppCodeWrapper
-      else ObjectCodeWrapper
+      if effectiveScalaVersion.startsWith("2") then
+        AppCodeWrapper(effectiveScalaVersion, logWarning)
+      else ObjectCodeWrapper(effectiveScalaVersion, logWarning)
 
     buildOptions.scriptOptions.forceObjectWrapper match {
       case Some(true) => objectCodeWrapperForScalaVersion
       case _ =>
         buildOptions.scalaOptions.platform.map(_.value) match {
-          case Some(_: Platform.JS.type)                  => objectCodeWrapperForScalaVersion
-          case _ if effectiveScalaVersion.startsWith("2") => AppCodeWrapper
-          case _                                          => ClassCodeWrapper
+          case Some(_: Platform.JS.type) => objectCodeWrapperForScalaVersion
+          case _ if effectiveScalaVersion.startsWith("2") =>
+            AppCodeWrapper(effectiveScalaVersion, logWarning)
+          case _ => ClassCodeWrapper(effectiveScalaVersion, logWarning)
         }
     }
   }