Enhance help message for language flag

Author: noti0na1

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

@@ -53,7 +53,7 @@ trait CliCommand:
   end distill
 
   /** Creates a help message for a subset of options based on cond */
-  protected def availableOptionsMsg(p: Setting[?] => Boolean)(using settings: ConcreteSettings)(using SettingsState): String =
+  protected def availableOptionsMsg(p: Setting[?] => Boolean, showArgFileMsg: Boolean = true)(using settings: ConcreteSettings)(using SettingsState): String =
     // result is (Option Name, descrption\ndefault: value\nchoices: x, y, z
     def help(s: Setting[?]): (String, String) =
       // For now, skip the default values that do not make sense for the end user, such as 'false' for the version command.
@@ -67,7 +67,10 @@ trait CliCommand:
     val ss = settings.allSettings.filter(p).toList.sortBy(_.name)
     val formatter = Columnator("", "", maxField = 30)
     val fresh = ContextBase().initialCtx.fresh.setSettings(summon[SettingsState])
-    formatter(List(ss.map(help) :+ ("@<file>", "A text file containing compiler arguments (options and source files).")))(using fresh)
+    var msg = ss.map(help)
+    if showArgFileMsg then
+      msg = msg :+ ("@<file>", "A text file containing compiler arguments (options and source files).")
+    formatter(List(msg))(using fresh)
   end availableOptionsMsg
 
   protected def shortUsage: String = s"Usage: $cmdName <options> <source files>"

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

@@ -9,7 +9,7 @@ abstract class CompilerCommand extends CliCommand:
 
   final def helpMsg(using settings: ConcreteSettings)(using SettingsState, Context): String =
     settings.allSettings.find(isHelping) match
-      case Some(s) => s.description
+      case Some(s) => availableOptionsMsg(_ == s, showArgFileMsg = false)
       case _ =>
         if (settings.help.value) usageMessage
         else if (settings.Vhelp.value) vusageMessage

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

@@ -38,6 +38,15 @@ object Feature:
     defn.languageExperimentalFeatures
       .map(sym => experimental(sym.name))
       .filterNot(_ == captureChecking) // TODO is this correct?
+      
+  val values = List(
+    nme.help,
+    nme.noAutoTupling, nme.dynamics, nme.unsafeNulls, nme.postfixOps, nme.strictEquality,
+    nme.implicitConversions, nme.adhocExtensions,
+    namedTypeArguments, genericNumberLiterals, scala2macros,
+    dependent, erasedDefinitions, symbolLiterals, fewerBraces, saferExceptions,
+    clauseInterleaving, pureFunctions, captureChecking, into
+  )
 
   /** Is `feature` enabled by by a command-line setting? The enabling setting is
    *

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[String]] = MultiStringSetting(RootSetting, "language", "feature", "Enable one or more language features.", aliases = List("--language"))
+  val language: Setting[List[String]] = MultiChoiceSetting(RootSetting, "language", "feature", "Enable one or more language features.", choices = ScalaSettingsProperties.supportedLanguageFeatures, 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

@@ -26,6 +26,9 @@ object ScalaSettingsProperties:
   def supportedSourceVersions: List[String] =
     SourceVersion.values.toList.map(_.toString)
 
+  def supportedLanguageFeatures: List[String] =
+    Feature.values.toList.map(_.toString)
+
   def defaultClasspath: String = sys.env.getOrElse("CLASSPATH", ".")
 
   def defaultPageWidth: Int = {

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

@@ -352,7 +352,7 @@ object Settings:
     def ChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: String, aliases: List[String] = Nil, legacyArgs: Boolean = false): Setting[String] =
       publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), aliases = aliases, legacyArgs = legacyArgs))
 
-    def MultiChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: List[String], aliases: List[String] = Nil): Setting[List[String]] =
+    def MultiChoiceSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[String], default: List[String] = Nil, aliases: List[String] = Nil): Setting[List[String]] =
       publish(Setting(category, prependName(name), descr, default, helpArg, Some(choices), aliases = aliases))
 
     def MultiChoiceHelpSetting(category: SettingCategory, name: String, helpArg: String, descr: String, choices: List[ChoiceWithHelp[String]], default: List[ChoiceWithHelp[String]], aliases: List[String] = Nil): Setting[List[ChoiceWithHelp[String]]] =

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -502,6 +502,7 @@ object StdNames {
     val _hashCode_ : N          = "_hashCode"
     val hash_ : N               = "hash"
     val head: N                 = "head"
+    val help: N                 = "help"
     val higherKinds: N          = "higherKinds"
     val idx: N                  = "idx"
     val identity: N             = "identity"