Fix scala#11774: only enable experimental features for snapshot and nightly

Author: odersky

compiler/src/dotty/tools/dotc/Driver.scala

@@ -77,6 +77,7 @@ class Driver {
     val ictx = rootCtx.fresh
     val summary = command.distill(args, ictx.settings)(ictx.settingsState)(using ictx)
     ictx.setSettings(summary.sstate)
+    Feature.checkExperimentalFlags(using ictx)
     MacroClassLoader.init(ictx)
     Positioned.init(using ictx)
 

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -279,6 +279,10 @@ trait TreeInfo[T >: Untyped <: Type] { self: Trees.Instance[T] =>
 
   def isLanguageImport(path: Tree): Boolean = languageImport(path).isDefined
 
+  def isExperimentalImport(path: Tree): Boolean = languageImport(path) match
+    case Some(nme.experimental) => true
+    case _ => false
+
   /** The underlying pattern ignoring any bindings */
   def unbind(x: Tree): Tree = unsplice(x) match {
     case Bind(_, y) => unbind(y)

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

@@ -28,7 +28,7 @@ object Feature:
   val symbolLiterals = deprecated("symbolLiterals")
   val fewerBraces = experimental("fewerBraces")
 
-/** Is `feature` enabled by by a command-line setting? The enabling setting is
+  /** Is `feature` enabled by by a command-line setting? The enabling setting is
    *
    *       -language:<prefix>feature
    *
@@ -97,4 +97,15 @@ object Feature:
     else
       false
 
+  val experimentalWarningMessage = "Experimental features may only be used with nightly or snapshot version of compiler."
+
+  /** Check that experimental compiler options are only set for snapshot or nightly compiler versions. */
+  def checkExperimentalFlags(using Context): Unit =
+    if !Properties.experimental then
+      val features = ctx.settings.language.value.filter(_.startsWith("experimental."))
+      if features.nonEmpty then
+        report.error(
+          i"""$experimentalWarningMessage
+             |The experimental language features were enabled via -language:$features%,%""")
+
 end Feature

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -32,6 +32,7 @@ import config.Feature
 import config.Feature.{sourceVersion, migrateTo3}
 import config.SourceVersion._
 import config.SourceVersion
+import config.Properties
 
 object Parsers {
 
@@ -3079,6 +3080,8 @@ object Parsers {
       val imp = Import(tree, selectors)
       if isLanguageImport(tree) then
         in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
+        if isExperimentalImport(tree) && !Properties.experimental then
+          report.error(Feature.experimentalWarningMessage, imp.srcPos)
         for
           case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors
           if allSourceVersionNames.contains(imported)