Drop @mixin entirely

Replace with @transparentTrait for now; that way we need not distinguish
between bootstrapped and non-bootstrapped code.

Author: odersky

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

@@ -912,7 +912,7 @@ class Definitions {
   @tu lazy val InvariantBetweenAnnot: ClassSymbol = requiredClass("scala.annotation.internal.InvariantBetween")
   @tu lazy val MainAnnot: ClassSymbol = requiredClass("scala.main")
   @tu lazy val MigrationAnnot: ClassSymbol = requiredClass("scala.annotation.migration")
-  @tu lazy val MixinAnnot: ClassSymbol = requiredClass("scala.annotation.mixin")
+  @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 SourceFileAnnot: ClassSymbol = requiredClass("scala.annotation.internal.SourceFile")
@@ -1507,8 +1507,8 @@ class Definitions {
   def isInfix(sym: Symbol)(using Context): Boolean =
     (sym eq Object_eq) || (sym eq Object_ne)
 
-  @tu lazy val assumedMixinTraits =
-    Set(ComparableClass, ProductClass, SerializableClass,
+  @tu lazy val assumedTransparentTraits =
+    Set[Symbol](ComparableClass, ProductClass, SerializableClass,
       // add these for now, until we had a chance to retrofit 2.13 stdlib
       // we should do a more through sweep through it then.
       requiredClass("scala.collection.SortedOps"),

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

@@ -1111,8 +1111,8 @@ object SymDenotations {
 
     final def isTransparentTrait(using Context): Boolean =
       isAllOf(TransparentTrait)
-      || isClass
-          && (hasAnnotation(defn.MixinAnnot) || defn.assumedMixinTraits.contains(symbol.asClass))
+      || defn.assumedTransparentTraits.contains(symbol)
+      || isClass && hasAnnotation(defn.TransparentTraitAnnot)
 
     /** The class containing this denotation which has the given effective name. */
     final def enclosingClassNamed(name: Name)(using Context): Symbol = {

library/src-non-bootstrapped/scala/annotation/mixin.scala

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

library/src-non-bootstrapped/scala/reflect/Enum.scala

@@ -1,6 +1,6 @@
 package scala.runtime
 
-transparent trait EnumValue extends Product, Serializable:
+@annotation.transparentTrait 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 =