Make into and allowConversions experimental

Author: odersky

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

@@ -28,6 +28,7 @@ object Feature:
   val symbolLiterals = deprecated("symbolLiterals")
   val fewerBraces = experimental("fewerBraces")
   val saferExceptions = experimental("saferExceptions")
+  val into = experimental("into")
 
   /** Is `feature` enabled by by a command-line setting? The enabling setting is
    *

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

@@ -1907,7 +1907,9 @@ object Parsers {
         core()
 
     private def maybeInto(tp: () => Tree) =
-      if in.isIdent(nme.into) && canStartTypeTokens.contains(in.lookahead.token)
+      if in.isIdent(nme.into)
+          && in.featureEnabled(Feature.into)
+          && canStartTypeTokens.contains(in.lookahead.token)
       then atSpan(in.skipToken()) { Into(tp()) }
       else tp()
 

library/src/scala/annotation/allowConversions.scala

@@ -1,8 +1,10 @@
 package scala.annotation
+import annotation.experimental
 
 /** An annotation on a parameter type that allows implicit conversions
  *  for its arguments. Intended for use by Scala 2, to annotate Scala 2
  *  libraries. Scala 3 uses the `convertibleTo` modifier on the parameter
  *  type instead.
  */
+@experimental
 class allowConversions extends scala.annotation.StaticAnnotation

library/src/scala/runtime/stdLibPatches/language.scala

@@ -60,6 +60,9 @@ object language:
     @compileTimeOnly("`saferExceptions` can only be used at compile time in import statements")
     object saferExceptions
 
+    @compileTimeOnly("`into` can only be used at compile time in import statements")
+    object into
+
   end experimental
 
   /** The deprecated object contains features that are no longer officially suypported in Scala.

tests/neg-custom-args/feature/convertible.scala

@@ -1,3 +1,5 @@
+import language.experimental.into
+
 class Text(val str: String)
 
 object Test:

tests/run-custom-args/fatal-warnings/convertible.scala

@@ -1,3 +1,5 @@
+import language.experimental.into
+
 class Text(val str: String)
 
 given Conversion[String, Text] = Text(_)

tests/run/Parser.scala

@@ -1,3 +1,5 @@
+import language.experimental.into
+
 type Input = List[String]
 
 trait ParseResult[+T]