Properly revert old behaviour of Xlint

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

@@ -360,7 +360,7 @@ private sealed trait XSettings:
   val XmacroSettings: Setting[List[String]] = MultiStringSetting(AdvancedSetting, "Xmacro-settings", "setting1,setting2,..settingN", "List of settings which exposed to the macros")
 
   @deprecated(message = "Superseded by -Wshadow, Scheduled for removal", since = "3.5.0")
-  val Xlint: Setting[_] = BooleanSetting(AdvancedSetting, "Xlint", "Enable or disable specific warnings", deprecation = Some(Deprecation("Use -Wshadow to enable shadowing lints. Scheduled for removal.")))
+  val Xlint: Setting[_] = BooleanSetting(AdvancedSetting, "Xlint", "Enable or disable specific warnings", deprecation = Some(Deprecation("Use -Wshadow to enable shadowing lints. Scheduled for removal.")), ignoreInvalidArgs = true)
 
 end XSettings
 

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

@@ -251,7 +251,11 @@ object Settings:
       def argValRest: String =
         if(prefix.isEmpty) arg.dropWhile(_ != ':').drop(1) else arg.drop(prefix.get.length)
 
-      if matches(arg) then doSet(argValRest)
+      if matches(arg) then
+        deprecation match
+          case Some(Deprecation(msg, _)) if ignoreInvalidArgs => // a special case for Xlint
+            state.deprecated(s"Option $name is deprecated: $msg")
+          case _ => doSet(argValRest)
       else state
 
     end tryToSet
@@ -367,8 +371,8 @@ object Settings:
       assert(!name.startsWith("-"), s"Setting $name cannot start with -")
       "-" + name
 
-    def BooleanSetting(category: SettingCategory, name: String, descr: String, initialValue: Boolean = false, aliases: List[String] = Nil, preferPrevious: Boolean = false, deprecation: Option[Deprecation] = None): Setting[Boolean] =
-      publish(Setting(category, prependName(name), descr, initialValue, aliases = aliases, preferPrevious = preferPrevious, deprecation = deprecation))
+    def BooleanSetting(category: SettingCategory, name: String, descr: String, initialValue: Boolean = false, aliases: List[String] = Nil, preferPrevious: Boolean = false, deprecation: Option[Deprecation] = None, ignoreInvalidArgs: Boolean = false): Setting[Boolean] =
+      publish(Setting(category, prependName(name), descr, initialValue, aliases = aliases, preferPrevious = preferPrevious, deprecation = deprecation, ignoreInvalidArgs = ignoreInvalidArgs))
 
     def StringSetting(category: SettingCategory, name: String, helpArg: String, descr: String, default: String, aliases: List[String] = Nil, deprecation: Option[Deprecation] = None): Setting[String] =
       publish(Setting(category, prependName(name), descr, default, helpArg, aliases = aliases, deprecation = deprecation))

compiler/test/dotty/tools/dotc/config/ScalaSettingsTests.scala

@@ -148,6 +148,17 @@ class ScalaSettingsTests:
       val conf = settings.processArguments(argSummary, processAll = true, skipped = Nil)
       assert(newSetting.isDefaultIn(conf.sstate), s"Setting $deprecatedArgument was forwarded to ${newSetting.name}, when it should be ignored because first option was erroreus")
 
+  // -Xlint was handled in a special way when it was added, making in hard to deprecate it.
+  // For now on we will retain old behavior, in next version we will emit deprecation warning.
+  // It is also scheduled for removal in future versions.
+  @Test def `Make Xlint to ignore invalid args`: Unit =
+    val settings = ScalaSettings
+    val args = List("-Xlint:-unused,_")
+    val argSummary = ArgsSummary(settings.defaultState, args, errors = Nil, warnings = Nil)
+    val conf = settings.processArguments(argSummary, processAll = true, skipped = Nil)
+    assert(conf.warnings.contains("Option -Xlint is deprecated: Use -Wshadow to enable shadowing lints. Scheduled for removal."))
+    assert(conf.errors.isEmpty)
+
   @nowarn("cat=deprecation")
   @Test def `Deprecated options aliases are correctly mapped to their replacements`: Unit =
     def createTestCase(oldSetting: Setting[_], newSetting: Setting[_], value: String = "") =