Make Memoize not depend on prepareForDefDef.

I see no good reason for this, as it creates tight coupling between `ifs` in two methods.

Author: DarkDimius

src/dotty/tools/dotc/transform/Memoize.scala

@@ -48,37 +48,29 @@ import Decorators._
     case _ =>
   }
 
-  override def prepareForDefDef(tree: DefDef)(implicit ctx: Context) = {
+  override def transformDefDef(tree: DefDef)(implicit ctx: Context, info: TransformerInfo): Tree = {
     val sym = tree.symbol
-    if (sym.isGetter && !sym.is(NoFieldNeeded)) {
+    lazy val field = {
       // allocate field early so that initializer has the right owner for subsequeny phases in
       // the group.
       val maybeMutable = if (sym is Stable) EmptyFlags else Mutable
-      val field = ctx.newSymbol(
+      val field = sym.field.orElse(ctx.newSymbol(
         owner = ctx.owner,
         name = sym.name.asTermName.fieldName,
         flags = Private | maybeMutable,
         info = sym.info.resultType,
-        coord = tree.pos).enteredAfter(thisTransform)
-      tree.rhs.changeOwnerAfter(sym, field, thisTransform)
-    }
-    this
-  }
+        coord = tree.pos).enteredAfter(thisTransform))
 
-  override def transformDefDef(tree: DefDef)(implicit ctx: Context, info: TransformerInfo): Tree = {
-    val sym = tree.symbol
-    def field = {
-      val field = sym.field.asTerm
-      assert(field.exists, i"no field for ${sym.showLocated} in ${sym.owner.info.decls.toList.map{_.showDcl}}%; %")
-      field
+      field.asTerm
     }
     if (sym.is(Accessor, butNot = NoFieldNeeded))
       if (sym.isGetter) {
-        val fieldDef = transformFollowing(ValDef(field, tree.rhs))
+        val fieldDef = transformFollowing(ValDef(field, tree.rhs.changeOwner(sym, field)))
         val getterDef = cpy.DefDef(tree)(rhs = transformFollowingDeep(ref(field)))
         Thicket(fieldDef, getterDef)
       }
       else if (sym.isSetter) {
+        // @odersky, next line does nothing(except potential MatchError). is this intended?
         if (!sym.is(ParamAccessor)) { val Literal(Constant(())) = tree.rhs }
         val initializer = Assign(ref(field), ref(tree.vparamss.head.head.symbol))
         cpy.DefDef(tree)(rhs = transformFollowingDeep(initializer))