Standardize how previously experimental features are handled

If a feature was previously experimental and is now standard, we change
any tests for that feature to be only dependent on the source version where
the feature was standardized. Language imports in old source versions
will no longer enable the feature. (And these language imports
also come with a deprecation message).

If a feature was previously experimental and is now dropped, the feature
becomes unavailable also in old versions.

The motivation to do it this way is to insist that experimental features
are ephemeral. We should not be able to rely on an experimental feature
forever in an old version.

This commit implements this policy for fewerBraces and clauseInterleaving.
Two implemented extensions (relaxedExtensionImports, betterMatchTypeExtractors)
already implemented it before.

Author: odersky

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

@@ -28,7 +28,6 @@ object Feature:
   val dependent = experimental("dependent")
   val erasedDefinitions = experimental("erasedDefinitions")
   val symbolLiterals = deprecated("symbolLiterals")
-  val fewerBraces = experimental("fewerBraces")
   val saferExceptions = experimental("saferExceptions")
   val clauseInterleaving = experimental("clauseInterleaving")
   val pureFunctions = experimental("pureFunctions")
@@ -60,9 +59,7 @@ object Feature:
     (dependent, "Allow dependent method types"),
     (erasedDefinitions, "Allow erased definitions"),
     (symbolLiterals, "Allow symbol literals"),
-    (fewerBraces, "Enable support for using indentation for arguments"),
     (saferExceptions, "Enable safer exceptions"),
-    (clauseInterleaving, "Enable clause interleaving"),
     (pureFunctions, "Enable pure functions for capture checking"),
     (captureChecking, "Enable experimental capture checking"),
     (into, "Allow into modifier on parameter types"),
@@ -124,9 +121,6 @@ object Feature:
 
   def namedTypeArgsEnabled(using Context) = enabled(namedTypeArguments)
 
-  def clauseInterleavingEnabled(using Context) =
-    sourceVersion.isAtLeast(`3.6`) || enabled(clauseInterleaving)
-
   def betterForsEnabled(using Context) = enabled(betterFors)
 
   def genericNumberLiteralsEnabled(using Context) = enabled(genericNumberLiterals)
@@ -169,9 +163,6 @@ object Feature:
   def migrateTo3(using Context): Boolean =
     sourceVersion == `3.0-migration`
 
-  def fewerBracesEnabled(using Context) =
-    sourceVersion.isAtLeast(`3.3`) || enabled(fewerBraces)
-
   /** If current source migrates to `version`, issue given warning message
    *  and return `true`, otherwise return `false`.
    */

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

@@ -28,6 +28,11 @@ enum SourceVersion:
 
   def isAtMost(v: SourceVersion) = stable.ordinal <= v.ordinal
 
+  def enablesFewerBraces = isAtLeast(`3.3`)
+  def enablesClauseInterleaving = isAtLeast(`3.6`)
+  def enablesNewGivens = isAtLeast(`3.6`)
+  def enablesNamedTuples = isAtLeast(`3.6`)
+
 object SourceVersion extends Property.Key[SourceVersion]:
   def defaultSourceVersion = `3.6`
 

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

@@ -868,7 +868,7 @@ object Parsers {
         }
       })
       canRewrite &= (in.isAfterLineEnd || statCtdTokens.contains(in.token)) // test (5)
-      if canRewrite && (!underColonSyntax || Feature.fewerBracesEnabled) then
+      if canRewrite && (!underColonSyntax || sourceVersion.enablesFewerBraces) then
         val openingPatchStr =
           if !colonRequired then ""
           else if testChar(startOpening - 1, Chars.isOperatorPart(_)) then " :"
@@ -1119,7 +1119,7 @@ object Parsers {
      *      body
      */
     def isColonLambda =
-      Feature.fewerBracesEnabled && in.token == COLONfollow && followingIsLambdaAfterColon()
+      sourceVersion.enablesFewerBraces && in.token == COLONfollow && followingIsLambdaAfterColon()
 
     /**   operand { infixop operand | MatchClause } [postfixop],
      *
@@ -3914,7 +3914,7 @@ object Parsers {
         val ident = termIdent()
         var name = ident.name.asTermName
         val paramss =
-          if Feature.clauseInterleavingEnabled(using in.languageImportContext) then
+          if sourceVersion.enablesClauseInterleaving then
             typeOrTermParamClauses(ParamOwner.Def, numLeadParams)
           else
             val tparams = typeParamClauseOpt(ParamOwner.Def)

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

@@ -17,7 +17,7 @@ import scala.collection.mutable
 import scala.collection.immutable.SortedMap
 import rewrites.Rewrites.patch
 import config.Feature
-import config.Feature.{migrateTo3, fewerBracesEnabled}
+import config.Feature.migrateTo3
 import config.SourceVersion.{`3.0`, `3.0-migration`}
 import config.MigrationVersion
 import reporting.{NoProfile, Profile, Message}
@@ -663,7 +663,7 @@ object Scanners {
         if token == COLONop && inTemplate then
           report.deprecationWarning(em"`:` after symbolic operator is deprecated; use backticks around operator instead", sourcePos(offset))
           true
-        else token == COLONfollow && (inTemplate || fewerBracesEnabled)
+        else token == COLONfollow && (inTemplate || sourceVersion.enablesFewerBraces)
       if enabled then
         peekAhead()
         val atEOL = isAfterLineEnd || token == EOF

tests/pos/interleavingExperimental.scala

@@ -1,5 +1,3 @@
-//> using options --source 3.5
-
-import scala.language.experimental.clauseInterleaving
+//> using options --source 3.6
 
 def ba[A](x: A)[B](using B): B = summon[B]