chore: deprecate scala.annotation.transparentTrait (#23116)

There is no use case for this annotation and transparent traits/classes
should use the `transparent` modifier.

Author: odersky

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

@@ -1048,7 +1048,6 @@ class Definitions {
   @tu lazy val NowarnAnnot: ClassSymbol = requiredClass("scala.annotation.nowarn")
   @tu lazy val UnusedAnnot: ClassSymbol = requiredClass("scala.annotation.unused")
   @tu lazy val UnrollAnnot: ClassSymbol = requiredClass("scala.annotation.unroll")
-  @tu lazy val TransparentTraitAnnot: ClassSymbol = requiredClass("scala.annotation.transparentTrait")
   @tu lazy val NativeAnnot: ClassSymbol = requiredClass("scala.native")
   @tu lazy val RepeatedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Repeated")
   @tu lazy val RuntimeCheckedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.RuntimeChecked")

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

@@ -1220,9 +1220,7 @@ object SymDenotations {
       || accessBoundary(defn.RootClass).isProperlyContainedIn(symbol.topLevelClass)
 
     final def isTransparentClass(using Context): Boolean =
-      is(TransparentType)
-      || defn.isAssumedTransparent(symbol)
-      || isClass && hasAnnotation(defn.TransparentTraitAnnot)
+      is(TransparentType) || defn.isAssumedTransparent(symbol)
 
     /** The class containing this denotation which has the given effective name. */
     final def enclosingClassNamed(name: Name)(using Context): Symbol = {

docs/_docs/reference/other-new-features/transparent-traits.md

@@ -59,7 +59,7 @@ will have type `Int | String` instead of the widened type `Any`.
 
 ## Which Traits and Classes Are Transparent?
 
-Traits and classes are declared transparent by adding the modifier `transparent`. Scala 2 traits and classes can also be declared transparent by adding a [`@transparentTrait`](https://scala-lang.org/api/3.x/scala/annotation/transparentTrait.html) annotation. This annotation is defined in [`scala.annotation`](https://scala-lang.org/api/3.x/scala/annotation.html). It will be deprecated and phased out once Scala 2/3 interoperability is no longer needed.
+Traits and classes are declared transparent by adding the modifier `transparent`.
 
 The following classes and traits are automatically treated as transparent:
 ```scala

library/src/scala/annotation/transparentTrait.scala

@@ -5,4 +5,5 @@ package scala.annotation
  *  are not inferred when combined with other types in an intersection.
  *  See reference/other-new-features/transparent-traits.html for details.
  */
+@deprecated(message = "Transparent traits/classes via annotations is no longer supported. Use instead the `transparent` modifier", since = "3.8.0")
 final class transparentTrait extends StaticAnnotation

library/src/scala/reflect/Enum.scala

@@ -1,7 +1,7 @@
 package scala.reflect
 
 /** A base trait of all Scala enum definitions */
-@annotation.transparentTrait trait Enum extends Any, Product, Serializable:
+transparent trait Enum extends Any, Product, Serializable:
 
   /** A number uniquely identifying a case of an enum */
   def ordinal: Int

library/src/scala/runtime/EnumValue.scala

@@ -1,6 +1,6 @@
 package scala.runtime
 
-@annotation.transparentTrait trait EnumValue extends Product, Serializable:
+transparent trait EnumValue extends Product, Serializable:
   override def canEqual(that: Any) = this eq that.asInstanceOf[AnyRef]
   override def productArity: Int = 0
   override def productElement(n: Int): Any =