Accept legacy language features (#20454)

Fix #20419

* Accept legacy language features (from Scala 2),
* For other unknown features, it will still fail.

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -69,6 +69,13 @@ object Feature:
     (betterMatchTypeExtractors, "Enable better match type extractors")
   )
 
+  // legacy language features from Scala 2 that are no longer supported.
+  val legacyFeatures = List(
+    "higherKinds",
+    "existentials",
+    "reflectiveCalls"
+  )
+
   private def enabledLanguageFeaturesBySetting(using Context): List[String] =
     ctx.settings.language.value.asInstanceOf
 

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -114,7 +114,7 @@ trait CommonScalaSettings:
   val explainTypes: Setting[Boolean] = BooleanSetting(RootSetting, "explain-types", "Explain type errors in more detail (deprecated, use -explain instead).", aliases = List("--explain-types", "-explaintypes"))
   val explainCyclic: Setting[Boolean] = BooleanSetting(RootSetting, "explain-cyclic", "Explain cyclic reference errors in more detail.", aliases = List("--explain-cyclic"))
   val unchecked: Setting[Boolean] = BooleanSetting(RootSetting, "unchecked", "Enable additional warnings where generated code depends on assumptions.", initialValue = true, aliases = List("--unchecked"))
-  val language: Setting[List[ChoiceWithHelp[String]]] = MultiChoiceHelpSetting(RootSetting, "language", "feature", "Enable one or more language features.", choices = ScalaSettingsProperties.supportedLanguageFeatures, default = Nil, aliases = List("--language"))
+  val language: Setting[List[ChoiceWithHelp[String]]] = MultiChoiceHelpSetting(RootSetting, "language", "feature", "Enable one or more language features.", choices = ScalaSettingsProperties.supportedLanguageFeatures, legacyChoices = ScalaSettingsProperties.legacyLanguageFeatures, default = Nil, aliases = List("--language"))
   val experimental: Setting[Boolean] = BooleanSetting(RootSetting, "experimental", "Annotate all top-level definitions with @experimental. This enables the use of experimental features anywhere in the project.")
 
   /* Coverage settings */

compiler/src/dotty/tools/dotc/config/ScalaSettingsProperties.scala

@@ -30,6 +30,9 @@ object ScalaSettingsProperties:
   def supportedLanguageFeatures: List[ChoiceWithHelp[String]] =
     Feature.values.map((n, d) => ChoiceWithHelp(n.toString, d))
 
+  val legacyLanguageFeatures: List[String] =
+    Feature.legacyFeatures
+
   def defaultClasspath: String = sys.env.getOrElse("CLASSPATH", ".")
 
   def defaultPageWidth: Int = {

compiler/src/dotty/tools/dotc/config/Settings.scala

@@ -85,7 +85,9 @@ object Settings:
     propertyClass: Option[Class[?]] = None,
     deprecation: Option[Deprecation] = None,
     // kept only for -Xkind-projector option compatibility
-    legacyArgs: Boolean = false)(private[Settings] val idx: Int):
+    legacyArgs: Boolean = false,
+    // accept legacy choices (for example, valid in Scala 2 but no longer supported)
+    legacyChoices: Option[Seq[?]] = None)(private[Settings] val idx: Int):
 
     validateSettingString(prefix.getOrElse(name))
     aliases.foreach(validateSettingString)
@@ -206,9 +208,14 @@ object Settings:
 
       def appendList(strings: List[String], argValue: String, args: List[String]) =
         choices match
-          case Some(valid) => strings.filterNot(valid.contains) match
-            case Nil => update(strings, argValue, args)
-            case invalid => invalidChoices(invalid)
+          case Some(valid) => strings.partition(valid.contains) match
+            case (_, Nil) => update(strings, argValue, args)
+            case (validStrs, invalidStrs) => legacyChoices match
+              case Some(validBefore) =>
+                invalidStrs.filterNot(validBefore.contains) match
+                  case Nil => update(validStrs, argValue, args)
+                  case realInvalidStrs => invalidChoices(realInvalidStrs)
+              case _ => invalidChoices(invalidStrs)
           case _ => update(strings, argValue, args)
 
       def doSet(argRest: String) =
@@ -380,11 +387,11 @@ object Settings:
     def ChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: String, aliases: List[String] = Nil, legacyArgs: Boolean = false, deprecation: Option[Deprecation] = None): Setting[String] =
       publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), aliases = aliases, legacyArgs = legacyArgs, deprecation = deprecation))
 
-    def MultiChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: List[String] = Nil, aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[List[String]] =
-      publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), aliases = aliases, deprecation = deprecation))
+    def MultiChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: List[String] = Nil, legacyChoices: List[String] = Nil, aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[List[String]] =
+      publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), legacyChoices = Some(legacyChoices), aliases = aliases, deprecation = deprecation))
 
-    def MultiChoiceHelpSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[ChoiceWithHelp[String]], default: List[ChoiceWithHelp[String]], aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[List[ChoiceWithHelp[String]]] =
-      publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), aliases = aliases, deprecation = deprecation))
+    def MultiChoiceHelpSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[ChoiceWithHelp[String]], default: List[ChoiceWithHelp[String]], legacyChoices: List[String] = Nil, aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[List[ChoiceWithHelp[String]]] =
+      publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), legacyChoices = Some(legacyChoices), aliases = aliases, deprecation = deprecation))
 
     def IntSetting(category: SettingCategory, name: String, descr: String, default: Int, aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[Int] =
       publish(Setting(category, prependName(name), descr, default, aliases = aliases, deprecation = deprecation))

compiler/test/dotty/tools/dotc/SettingsTests.scala

@@ -272,8 +272,8 @@ class SettingsTests {
       val booleanSetting = BooleanSetting(RootSetting, "booleanSetting", "booleanSetting", false)
       val stringSetting  = StringSetting(RootSetting, "stringSetting", "stringSetting", "", "test")
       val choiceSetting =  ChoiceSetting(RootSetting, "choiceSetting", "choiceSetting", "", List("a", "b"), "a")
-      val multiChoiceSetting=  MultiChoiceSetting(RootSetting, "multiChoiceSetting", "multiChoiceSetting", "", List("a", "b"), List())
-      val multiChoiceHelpSetting=  MultiChoiceHelpSetting(RootSetting, "multiChoiceHelpSetting", "multiChoiceHelpSetting", "", List(ChoiceWithHelp("a", "a"), ChoiceWithHelp("b", "b")), List())
+      val multiChoiceSetting=  MultiChoiceSetting(RootSetting, "multiChoiceSetting", "multiChoiceSetting", "", List("a", "b"), List(), legacyChoices = List("c"))
+      val multiChoiceHelpSetting=  MultiChoiceHelpSetting(RootSetting, "multiChoiceHelpSetting", "multiChoiceHelpSetting", "", List(ChoiceWithHelp("a", "a"), ChoiceWithHelp("b", "b")), List(), legacyChoices = List("c"))
       val intSetting = IntSetting(RootSetting, "intSetting", "intSetting", 0)
       val intChoiceSetting = IntChoiceSetting(RootSetting, "intChoiceSetting", "intChoiceSetting", List(1,2,3), 1)
       val multiStringSetting = MultiStringSetting(RootSetting, "multiStringSetting", "multiStringSetting", "", List("a", "b"), List())
@@ -289,8 +289,8 @@ class SettingsTests {
         List("-booleanSetting", "true"),
         List("-stringSetting", "newTest"),
         List("-choiceSetting", "b"),
-        List("-multiChoiceSetting", "a,b"),
-        List("-multiChoiceHelpSetting", "a,b"),
+        List("-multiChoiceSetting", "a,b,c"),
+        List("-multiChoiceHelpSetting", "a,b,c"),
         List("-intSetting", "42"),
         List("-intChoiceSetting", "2"),
         List("-multiStringSetting", "a,b"),