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.

* A class is experimental if
  * It is annotated with `@experimental`
  * It is a nested class of an experimental class. Annotation `@experimental` is inferred.
  * It extends an experimental class. An error is emitted if it does not have @experimental.
* A member definition is experimental if
  * It is annotated with `@experimental`
  * Its a member of an experimental class
* `class experimental` is experimental

Author: nicolasstucki

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 $which may only be used with a nightly or snapshot version of the compiler", srcPos)
+
+  def checkExperimentalDef(sym: Symbol, srcPos: SrcPos)(using Context) =
+    if !isExperimentalEnabled then
+      report.error(i"$sym is marked @experimental and therefore may only be used with a nightly or snapshot version of the 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

@@ -909,6 +909,7 @@ 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

@@ -3084,7 +3084,7 @@ 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"
@@ -263,6 +265,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
                ctx
           super.transform(tree)(using gadtCtx)
         case tree: Ident if !tree.isType =>
+          Experimental.checkExperimental(tree)
           if tree.symbol.is(Inline) && !Inliner.inInlineMethod then
             ctx.compilationUnit.needsInlining = true
           checkNoConstructorProxy(tree)
@@ -271,6 +274,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
             case _ => tree
           }
         case tree @ Select(qual, name) =>
+          Experimental.checkExperimental(tree)
           if tree.symbol.is(Inline) then
             ctx.compilationUnit.needsInlining = true
           if (name.isTypeName) {
@@ -353,6 +357,7 @@ class PostTyper extends MacroTransform with IdentityDenotTransformer { thisPhase
           val sym = tree.symbol
           if (sym.isClass)
             VarianceChecker.check(tree)
+            Experimental.annotateExperimental(sym)
             // Add SourceFile annotation to top-level classes
             if sym.owner.is(Package)
                && ctx.compilationUnit.source.exists
@@ -388,6 +393,7 @@ 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 +259,16 @@ object SymUtils:
         && self.owner.linkedClass.is(Case)
         && self.owner.linkedClass.isDeclaredInfix
 
+    /** Is symbol declared or inherits @experimental? */
+    def isExperimental(using Context): Boolean =
+      // TODO should be add `@experimental` to `class experimental` in PostTyper?
+      self.eq(defn.ExperimentalAnnot)
+      || self.hasAnnotation(defn.ExperimentalAnnot)
+      || (self.maybeOwner.isClass && self.owner.hasAnnotation(defn.ExperimentalAnnot))
+
+    def isNestedInExperimental(using Context): Boolean =
+      self.ownersIterator.drop(1).exists(_.hasAnnotation(defn.ExperimentalAnnot))
+
     /** The declared self type of this class, as seen from `site`, stripping
     *  all refinements for opaque types.
     */

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
@@ -93,6 +94,7 @@ 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

@@ -212,6 +212,7 @@ 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.
@@ -477,6 +478,8 @@ 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()
     }
@@ -1136,6 +1139,15 @@ object RefChecks {
 
   end checkImplicitNotFoundAnnotation
 
+
+  /** Check that classes extending experimental classes or nested in experimental classes have the @experimental annotation. */
+  private def checkExperimentalInheritance(cls: ClassSymbol)(using Context): Unit =
+    if !cls.hasAnnotation(defn.ExperimentalAnnot) then
+      cls.info.parents.find(_.typeSymbol.isExperimental) match
+        case Some(parent) =>
+          report.error(em"extension of experimental ${parent.typeSymbol} must have @experimental annotation", cls.srcPos)
+        case _ =>
+  end checkExperimentalInheritance
 }
 import RefChecks._
 
@@ -1224,6 +1236,7 @@ class RefChecks extends MiniPhase { thisPhase =>
     checkCompanionNameClashes(cls)
     checkAllOverrides(cls)
     checkImplicitNotFoundAnnotation.template(cls.classDenot)
+    checkExperimentalInheritance(cls)
     tree
   }
   catch {

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

@@ -0,0 +1,45 @@
+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
+      && !ctx.owner.isExperimental // already reported on the @experimental of the owner
+      && !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)
+
+  def annotateExperimental(sym: Symbol)(using Context): Unit =
+    if sym.isClass && !sym.hasAnnotation(defn.ExperimentalAnnot) then
+      // Add @experimental annotation to all classes nested in an experimental definition
+      if !sym.owner.is(Package) && sym.isNestedInExperimental then
+        sym.addAnnotation(defn.ExperimentalAnnot)
+
+      // Add @experimental annotation to enum class definitions
+      val compSym = sym.companionClass
+      if compSym.is(Enum) && compSym.hasAnnotation(defn.ExperimentalAnnot) then
+        sym.addAnnotation(defn.ExperimentalAnnot)
+        sym.companionModule.addAnnotation(defn.ExperimentalAnnot)

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/util/FromDigits.scala

@@ -3,6 +3,7 @@ import scala.math.{BigInt}
 import quoted._
 import annotation.internal.sharable
 
+
 /** A type class for types that admit numeric literals.
  */
 trait FromDigits[T] {
@@ -28,7 +29,7 @@ object FromDigits {
   trait WithRadix[T] extends FromDigits[T] {
     def fromDigits(digits: String): T = fromDigits(digits, 10)
 
-    /** Convert digits string with given radix to numberof type `T`.
+    /** Convert digits string with given radix to number of type `T`.
      *  E.g. if radix is 16, digits `a..f` and `A..F` are also allowed.
      */
     def fromDigits(digits: String, radix: Int): T

tests/neg-custom-args/no-experimental/experimentalAnnotation.scala

@@ -0,0 +1,103 @@
+import scala.annotation.experimental
+
+@experimental // error
+val x = ()
+
+@experimental // error
+def f() = ()
+
+@experimental // error
+class A:
+  def f() = 1
+
+@experimental // error
+class B extends A:
+  override def f() = 2
+
+@experimental // error
+type X
+
+@experimental // error
+type Y = Int
+
+@experimental // error
+opaque type Z = Int
+
+@experimental // error
+object X:
+  def fx() = 1
+
+class C:
+  @experimental // error
+  def f() = 1
+
+class D extends C:
+  override def f() = 2
+
+trait A2:
+  @experimental // error
+  def f(): Int
+
+trait B2:
+  def f(): Int
+
+class C2 extends A2, B2:
+  def f(): Int = 1
+
+object Extractor1:
+  def unapply(s: Any): Option[A] = ??? // error
+
+object Extractor2:
+  @experimental // error
+  def unapply(s: Any): Option[Int] = ???
+
+
+@experimental // error
+trait ExpSAM {
+  def foo(x: Int): Int
+}
+def bar(f: ExpSAM): Unit = {} // error
+
+@experimental // error
+enum E:
+  case A
+  case B
+
+def test(
+  p1: A, // error
+  p2: List[A], // error
+  p3: X, // error
+  p4: Y, // error
+  p5: Z, // error
+): Unit =
+  f() // error
+  x // error
+  new A // error
+  new B // error
+  X.fx() // error
+  import X.fx // error
+  fx() // error
+  val i1 = identity[X] // error // error
+  val i2 = identity[A] // error // error
+  val a: A = ??? // error
+  val b: B = ??? // error
+  val c: C = ???
+  val d: D = ???
+  val c2: C2 = ???
+  a.f() // error
+  b.f() // error
+  c.f() // error
+  d.f() // ok because D.f is a stable API
+  c2.f() // ok because B2.f is a stable API
+  ()
+
+  (??? : Any) match
+    case _: A => // error // error
+    case Extractor1(_) => // error
+    case Extractor2(_) => // error
+
+  bar(x => x) // error
+
+  E.A // error
+  E.B // error
+  val e: E = ??? // error

tests/neg-custom-args/no-experimental/experimentalExperimental.scala

@@ -0,0 +1 @@
+class MyExperimentalAnnot extends scala.annotation.experimental // error

tests/neg-custom-args/no-experimental/experimentalnline.scala

@@ -0,0 +1,8 @@
+import scala.annotation.experimental
+
+@experimental // error
+inline def g() = ()
+
+def test: Unit =
+  g() // errors
+  ()

tests/neg/experimentalExperimental.scala

@@ -0,0 +1 @@
+class MyExperimentalAnnot extends scala.annotation.experimental // error