Add superTrait annotation

This can be added in a backwards compatible way to cross-compiling sources

Author: odersky

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

@@ -803,6 +803,7 @@ class Definitions {
   @tu lazy val ScalaStrictFPAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.strictfp")
   @tu lazy val ScalaStaticAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.static")
   @tu lazy val SerialVersionUIDAnnot: ClassSymbol = ctx.requiredClass("scala.SerialVersionUID")
+  @tu lazy val SuperTraitAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.superTrait")
   @tu lazy val TASTYSignatureAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.internal.TASTYSignature")
   @tu lazy val TASTYLongSignatureAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.internal.TASTYLongSignature")
   @tu lazy val TailrecAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.tailrec")

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

@@ -1168,7 +1168,9 @@ object SymDenotations {
 
     final def isSuperTrait(using Context): Boolean =
       isClass
-      && (is(SuperTrait) || defn.assumedSuperTraits.contains(symbol.asClass))
+      && (is(SuperTrait)
+          || defn.assumedSuperTraits.contains(symbol.asClass)
+          || hasAnnotation(defn.SuperTraitAnnot))
 
     /** The class containing this denotation which has the given effective name. */
     final def enclosingClassNamed(name: Name)(implicit ctx: Context): Symbol = {

library/src/scala/annotation/superTrait.scala

@@ -0,0 +1,8 @@
+package scala.annotation
+
+/** Equivalent to declaring a super trait in Scala 3. This annotation
+ *  should be used only for files that need to cross-compile between
+ *  Scala 2 and 3.
+ */
+final class superTrait extends StaticAnnotation
+