Add @experimental annotation

The `@exerimental` annotation marks definitions as _experimental_ feature.
These can be used in the same situattions where `languange.experimental` can be used.

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

@@ -99,20 +99,24 @@ object Feature:
 
   private val assumeExperimentalIn = Set("dotty.tools.vulpix.ParallelTesting")
 
-  def checkExperimentalFeature(which: String, srcPos: SrcPos = NoSourcePosition)(using Context) =
-    def hasSpecialPermission =
-      new Exception().getStackTrace.exists(elem =>
-        assumeExperimentalIn.exists(elem.getClassName().startsWith(_)))
-    if !(Properties.experimental || hasSpecialPermission)
-       || ctx.settings.YnoExperimental.value
-    then
-      //println(i"${new Exception().getStackTrace.map(_.getClassName).toList}%\n%")
-      report.error(i"Experimental feature$which may only be used with nightly or snapshot version of compiler", srcPos)
+  def checkExperimentalFeature(which: String, srcPos: SrcPos)(using Context) =
+    if !isExperimentalEnabled then
+      report.error(i"Experimental $which may only be used with nightly or snapshot version of compiler", srcPos)
+
+  def checkExperimentalDef(sym: Symbol, srcPos: SrcPos)(using Context) =
+    if !isExperimentalEnabled then
+      report.error(i"Experimental $sym may only be used with nightly or snapshot version of compiler", srcPos)
 
   /** Check that experimental compiler options are only set for snapshot or nightly compiler versions. */
   def checkExperimentalSettings(using Context): Unit =
     for setting <- ctx.settings.language.value
         if setting.startsWith("experimental.") && setting != "experimental.macros"
-    do checkExperimentalFeature(s" $setting")
+    do checkExperimentalFeature(s"feature $setting", NoSourcePosition)
+
+  def isExperimentalEnabled(using Context): Boolean =
+    def hasSpecialPermission =
+      Thread.currentThread.getStackTrace.exists(elem =>
+        assumeExperimentalIn.exists(elem.getClassName().startsWith(_)))
+    (Properties.experimental || hasSpecialPermission) && !ctx.settings.YnoExperimental.value
 
 end Feature

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

@@ -910,6 +910,7 @@ class Definitions {
   @tu lazy val ConstructorOnlyAnnot: ClassSymbol = requiredClass("scala.annotation.constructorOnly")
   @tu lazy val CompileTimeOnlyAnnot: ClassSymbol = requiredClass("scala.annotation.compileTimeOnly")
   @tu lazy val SwitchAnnot: ClassSymbol = requiredClass("scala.annotation.switch")
+  @tu lazy val ExperimentalAnnot: ClassSymbol = requiredClass("scala.annotation.experimental")
   @tu lazy val ThrowsAnnot: ClassSymbol = requiredClass("scala.throws")
   @tu lazy val TransientAnnot: ClassSymbol = requiredClass("scala.transient")
   @tu lazy val UncheckedAnnot: ClassSymbol = requiredClass("scala.unchecked")

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

@@ -3083,7 +3083,7 @@ object Parsers {
           if prefix == nme.experimental
              && selectors.exists(sel => Feature.experimental(sel.name) != Feature.scala2macros)
           then
-            Feature.checkExperimentalFeature("s", imp.srcPos)
+            Feature.checkExperimentalFeature("features", imp.srcPos)
           for
             case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors
             if allSourceVersionNames.contains(imported)

compiler/src/dotty/tools/dotc/plugins/Plugins.scala

@@ -3,7 +3,7 @@ package plugins
 
 import core._
 import Contexts._
-import config.{ PathResolver, Properties }
+import config.{ PathResolver, Feature }
 import dotty.tools.io._
 import Phases._
 import config.Printers.plugins.{ println => debug }
@@ -125,7 +125,7 @@ trait Plugins {
     val updatedPlan = Plugins.schedule(plan, pluginPhases)
 
     // add research plugins
-    if (Properties.experimental)
+    if (Feature.isExperimentalEnabled)
       plugins.collect { case p: ResearchPlugin => p }.foldRight(updatedPlan) {
         (plug, plan) => plug.init(options(plug), plan)
       }

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -14,6 +14,8 @@ import Symbols._, SymUtils._, NameOps._
 import ContextFunctionResults.annotateContextResults
 import config.Printers.typr
 import reporting._
+import util.Experimental
+
 
 object PostTyper {
   val name: String = "posttyper"
@@ -257,15 +259,19 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
 
     override def transform(tree: Tree)(using Context): Tree =
       try tree match {
-        case tree: Ident if !tree.isType =>
-          if tree.symbol.is(Inline) && !Inliner.inInlineMethod then
-            ctx.compilationUnit.needsInlining = true
-          checkNoConstructorProxy(tree)
-          tree.tpe match {
-            case tpe: ThisType => This(tpe.cls).withSpan(tree.span)
-            case _ => tree
-          }
+        case tree: Ident =>
+          Experimental.checkExperimental(tree)
+          if tree.isType then super.transform(tree)
+          else
+            if tree.symbol.is(Inline) && !Inliner.inInlineMethod then
+              ctx.compilationUnit.needsInlining = true
+            checkNoConstructorProxy(tree)
+            tree.tpe match {
+              case tpe: ThisType => This(tpe.cls).withSpan(tree.span)
+              case _ => tree
+            }
         case tree @ Select(qual, name) =>
+          Experimental.checkExperimental(tree)
           if tree.symbol.is(Inline) then
             ctx.compilationUnit.needsInlining = true
           if (name.isTypeName) {
@@ -382,6 +388,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           Checking.checkRealizable(ref.tpe, ref.srcPos)
           super.transform(tree)
         case tree: TypeTree =>
+          Experimental.checkExperimental(tree)
           tree.withType(
             tree.tpe match {
               case AnnotatedType(tpe, annot) => AnnotatedType(tpe, transformAnnot(annot))

compiler/src/dotty/tools/dotc/transform/SymUtils.scala

@@ -259,6 +259,13 @@ object SymUtils:
         && self.owner.linkedClass.is(Case)
         && self.owner.linkedClass.isDeclaredInfix
 
+    /** Is symbol declared experimental? */
+    def isExperimental(using Context): Boolean =
+      (self eq defn.ExperimentalAnnot)
+      || self.hasAnnotation(defn.ExperimentalAnnot)
+      || self.allOverriddenSymbols.nonEmpty && self.allOverriddenSymbols.forall(_.hasAnnotation(defn.ExperimentalAnnot)) // TODO infer @experimental?
+      || (self.maybeOwner.exists && self.owner.isClass && self.owner.hasAnnotation(defn.ExperimentalAnnot)) // TODO infer @experimental?
+
     /** The declared self type of this class, as seen from `site`, stripping
     *  all refinements for opaque types.
     */

compiler/src/dotty/tools/dotc/typer/Checking.scala

@@ -422,6 +422,14 @@ object Checking {
     }
   }
 
+  /** Check that classes extending experimental classes have the @experimental annotation */
+  def checkExperimentalInheritance(cls: ClassSymbol, parents: List[Type], srcPos: SrcPos)(using Context): Unit =
+    if !cls.hasAnnotation(defn.ExperimentalAnnot) then
+      parents.find(_.typeSymbol.isExperimental) match
+        case Some(parent) =>
+          report.error(em"extension of experimental ${parent.typeSymbol} must have @experimental annotation", srcPos)
+        case _ =>
+
   /** Check that symbol's definition is well-formed. */
   def checkWellFormed(sym: Symbol)(using Context): Unit = {
     def fail(msg: Message) = report.error(msg, sym.srcPos)

compiler/src/dotty/tools/dotc/typer/Inliner.scala

@@ -21,6 +21,7 @@ import Annotations.Annotation
 import SymDenotations.SymDenotation
 import Inferencing.isFullyDefined
 import config.Printers.inlining
+import config.Feature
 import ErrorReporting.errorTree
 import dotty.tools.dotc.util.{SimpleIdentityMap, SimpleIdentitySet, EqHashMap, SourceFile, SourcePosition, SrcPos}
 import dotty.tools.dotc.parsing.Parsers.Parser
@@ -92,6 +93,7 @@ object Inliner {
       if (tree.symbol == defn.CompiletimeTesting_typeChecks) return Intrinsics.typeChecks(tree)
       if (tree.symbol == defn.CompiletimeTesting_typeCheckErrors) return Intrinsics.typeCheckErrors(tree)
 
+    Feature.checkExperimentalDef(tree.symbol, tree)
 
     /** Set the position of all trees logically contained in the expansion of
      *  inlined call `call` to the position of `call`. This transform is necessary

compiler/src/dotty/tools/dotc/typer/RefChecks.scala

@@ -210,6 +210,7 @@ object RefChecks {
    *    1.9. If M is erased, O is erased. If O is erased, M is erased or inline.
    *    1.10.  If O is inline (and deferred, otherwise O would be final), M must be inline
    *    1.11.  If O is a Scala-2 macro, M must be a Scala-2 macro.
+   *    1.12.  If O is non-experimental, M must be non-experimental.
    *  2. Check that only abstract classes have deferred members
    *  3. Check that concrete classes do not have deferred definitions
    *     that are not implemented in a subclass.
@@ -475,6 +476,8 @@ object RefChecks {
           overrideError(i"needs to be declared with @targetName(${"\""}${other.targetName}${"\""}) so that external names match")
         else
           overrideError("cannot have a @targetName annotation since external names would be different")
+      else if !other.isExperimental && member.hasAnnotation(defn.ExperimentalAnnot) then // (1.12)
+        overrideError("may not override non-experimental member")
       else
         checkOverrideDeprecated()
     }

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2352,6 +2352,7 @@ class Typer extends Namer
       }
 
       checkNonCyclicInherited(cls.thisType, cls.info.parents, cls.info.decls, cdef.srcPos)
+      checkExperimentalInheritance(cls, cls.info.parents, cdef.srcPos)
 
       // check value class constraints
       checkDerivedValueClass(cls, body1)

compiler/src/dotty/tools/dotc/util/Experimental.scala

@@ -0,0 +1,32 @@
+package dotty.tools.dotc
+package util
+
+import dotty.tools.dotc.ast.tpd
+import dotty.tools.dotc.ast.Trees._
+import dotty.tools.dotc.config.Feature
+import dotty.tools.dotc.core.Contexts._
+import dotty.tools.dotc.core.Symbols._
+import dotty.tools.dotc.core.Types._
+import dotty.tools.dotc.core.Flags._
+import dotty.tools.dotc.transform.SymUtils._
+
+object Experimental:
+  import tpd._
+
+  def checkExperimental(tree: Tree)(using Context): Unit =
+    if tree.symbol.isExperimental
+      && !tree.symbol.isConstructor // already reported on the class
+      && !tree.symbol.is(ModuleClass) // already reported on the module
+      && (tree.span.exists || tree.symbol != defn.ExperimentalAnnot) // already reported on inferred annotations
+    then
+      Feature.checkExperimentalDef(tree.symbol, tree)
+
+  def checkExperimentalTypes(tree: Tree)(using Context): Unit =
+    val checker = new TypeTraverser:
+      def traverse(tp: Type): Unit =
+        if tp.typeSymbol.isExperimental then
+          Feature.checkExperimentalDef(tp.typeSymbol, tree)
+        else
+          traverseChildren(tp)
+    if !tree.span.isSynthetic then // avoid double errors
+      checker.traverse(tree.tpe)

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -240,7 +240,7 @@ class CompilationTests {
         Properties.compilerInterface, Properties.scalaLibrary, Properties.scalaAsm,
         Properties.dottyInterfaces, Properties.jlineTerminal, Properties.jlineReader,
       ).mkString(File.pathSeparator),
-      Array("-Ycheck-reentrant", "-language:postfixOps", "-Xsemanticdb", "-Yno-experimental")
+      Array("-Ycheck-reentrant", "-language:postfixOps", "-Xsemanticdb")
     )
 
     val libraryDirs = List(Paths.get("library/src"), Paths.get("library/src-bootstrapped"))

library/src/scala/annotation/experimental.scala

@@ -0,0 +1,14 @@
+package scala.annotation
+
+/** An annotation that can be used to mark a definition as experimental.
+ *
+ *  This class is experimental as well as if it was defined as
+ *  ```scala
+ *  @experimental
+ *  class experimental extends StaticAnnotation
+ *  ```
+ *
+ *  @syntax markdown
+ */
+// @experimental
+class experimental extends StaticAnnotation

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

@@ -18,6 +18,7 @@ object language:
    *
    *  @group experimental
    */
+  @scala.annotation.experimental
   object experimental:
 
     /* Experimental support for richer dependent types (disabled for now)

library/src/scala/util/FromDigits.scala

@@ -2,9 +2,12 @@ package scala.util
 import scala.math.{BigInt}
 import quoted._
 import annotation.internal.sharable
+import annotation.experimental
+
 
 /** A type class for types that admit numeric literals.
  */
+@experimental
 trait FromDigits[T] {
 
   /** Convert `digits` string to value of type `T`
@@ -20,11 +23,13 @@ trait FromDigits[T] {
   def fromDigits(digits: String): T
 }
 
+@experimental
 object FromDigits {
 
   /** A subclass of `FromDigits` that also allows to convert whole number literals
    *  with a radix other than 10
    */
+  @experimental
   trait WithRadix[T] extends FromDigits[T] {
     def fromDigits(digits: String): T = fromDigits(digits, 10)
 
@@ -37,28 +42,34 @@ object FromDigits {
   /** A subclass of `FromDigits` that also allows to convert number
    *  literals containing a decimal point ".".
    */
+  @experimental
   trait Decimal[T] extends FromDigits[T]
 
   /** A subclass of `FromDigits`that allows also to convert number
    *  literals containing a decimal point "." or an
    *  exponent `('e' | 'E')['+' | '-']digit digit*`.
    */
+  @experimental
   trait Floating[T] extends Decimal[T]
 
   /** The base type for exceptions that can be thrown from
    *  `fromDigits` conversions
    */
+  @experimental
   abstract class FromDigitsException(msg: String) extends NumberFormatException(msg)
 
   /** Thrown if value of result does not fit into result type's range */
+  @experimental
   class NumberTooLarge(msg: String = "number too large") extends FromDigitsException(msg)
 
   /** Thrown in case of numeric underflow (e.g. a non-zero
    *  floating point literal that produces a zero value)
    */
+  @experimental
   class NumberTooSmall(msg: String = "number too small") extends FromDigitsException(msg)
 
   /** Thrown if digit string is not legal for the given type */
+  @experimental
   class MalformedNumber(msg: String = "malformed number literal") extends FromDigitsException(msg)
 
   /** Convert digits and radix to integer value (either int or Long)
@@ -156,10 +167,12 @@ object FromDigits {
     x
   }
 
+  @experimental
   given BigIntFromDigits: WithRadix[BigInt] with {
     def fromDigits(digits: String, radix: Int): BigInt = BigInt(digits, radix)
   }
 
+  @experimental
   given BigDecimalFromDigits: Floating[BigDecimal] with {
     def fromDigits(digits: String): BigDecimal = BigDecimal(digits)
   }