@constructorOnly: better error message

This annotation used to be called @transientParam but that's no longer
the case.

Author: smarter

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

@@ -802,7 +802,7 @@ class Definitions {
   @tu lazy val TASTYLongSignatureAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.internal.TASTYLongSignature")
   @tu lazy val TailrecAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.tailrec")
   @tu lazy val ThreadUnsafeAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.threadUnsafe")
-  @tu lazy val TransientParamAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.constructorOnly")
+  @tu lazy val ConstructorOnlyAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.constructorOnly")
   @tu lazy val CompileTimeOnlyAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.compileTimeOnly")
   @tu lazy val SwitchAnnot: ClassSymbol = ctx.requiredClass("scala.annotation.switch")
   @tu lazy val ThrowsAnnot: ClassSymbol = ctx.requiredClass("scala.throws")

compiler/src/dotty/tools/dotc/transform/Constructors.scala

@@ -229,8 +229,8 @@ class Constructors extends MiniPhase with IdentityDenotTransformer { thisPhase =
       }
       else {
         val param = acc.subst(accessors, paramSyms)
-        if (param.hasAnnotation(defn.TransientParamAnnot))
-          ctx.error(em"transient parameter $acc is retained as field in class ${acc.owner}", acc.sourcePos)
+        if (param.hasAnnotation(defn.ConstructorOnlyAnnot))
+          ctx.error(em"${acc.name} is marked `@constructorOnly` but it is retained as a field in ${acc.owner}", acc.sourcePos)
         val target = if (acc.is(Method)) acc.field else acc
         if (!target.exists) Nil // this case arises when the parameter accessor is an alias
         else {