Add Erasure Preservation Phase

Author: HarrisL2

community-build/community-projects/cats

@@ -109,6 +109,7 @@ class Compiler {
          new TupleOptimizations,     // Optimize generic operations on tuples
          new LetOverApply,           // Lift blocks from receivers of applications
          new ArrayConstructors) ::   // Intercept creation of (non-generic) arrays and intrinsify.
+    List(new ErasurePreservation) :: //
     List(new Erasure) ::             // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
     List(new ElimErasedValueType,    // Expand erased value types to their underlying implementation types
          new PureStats,              // Remove pure stats from blocks

community-build/community-projects/intent

@@ -926,7 +926,18 @@ object Erasure {
      *  with more than `MaxImplementedFunctionArity` parameters to use a single
      *  parameter of type `[]Object`.
      */
-    override def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(using Context): Tree =
+    override def typedDefDef(ddef: untpd.DefDef, sym: Symbol)(using Context): Tree = // trace(i"typedDefDef $sym,"+s" ${atPhase(ctx.phaseId-1)(sym.info)}") {// Possible Annotation target
+      // atPhase(ctx.phaseId-1)(sym.info) match
+      //   case pt: PolyType =>
+      //     def toTypeB(tp: Type): TypeB = tp match
+      //       case tpr: TypeParamRef => TypeB.M(tpr.paramNum)
+      //       case _ => TypeB.None
+      //     pt.resType match
+      //       case mt: MethodType =>
+      //         sym.addAnnotation(ErasedInfo(pt.paramInfos.size, mt.paramInfos.map(toTypeB), toTypeB(mt.resType)))
+      //       case other =>
+      //         sym.addAnnotation(ErasedInfo(pt.paramInfos.size, Nil, toTypeB(other.widenExpr)))
+      //   case _ => ()
       if sym.isEffectivelyErased || sym.name.is(BodyRetainerName) then
         erasedDef(sym)
       else
@@ -970,6 +981,7 @@ object Erasure {
           tpt = untpd.TypedSplice(TypeTree(restpe).withSpan(ddef.tpt.span)),
           rhs = rhs1)
         super.typedDefDef(ddef1, sym)
+    // }
     end typedDefDef
 
     /** The outer parameter definition of a constructor if it needs one */

community-build/community-projects/utest

@@ -0,0 +1,61 @@
+package dotty.tools
+package dotc
+package transform
+
+import ast.Trees
+import core.Phases.*
+import core.DenotTransformers.*
+import core.Annotations.*
+import core.Denotations.*
+import core.SymDenotations.*
+import core.Symbols.*
+import core.Contexts.*
+import core.Types.*
+import core.Names.*
+import core.Constants.*
+import core.Decorators.*
+import typer.NoChecking
+import ast.{tpd, untpd}
+import reporting.*
+import dotty.tools.dotc.transform.MegaPhase.MiniPhase
+
+class ErasurePreservation extends MiniPhase {
+
+  override def phaseName: String = ErasurePreservation.name
+
+  override def description: String = ErasurePreservation.description
+
+  override def transformDefDef(tree: tpd.DefDef)(using Context): tpd.Tree =
+    val sym = tree.symbol
+    sym.info match
+        case pt: PolyType =>
+          def toTypeB(tp: Type): TypeB = tp match
+            case tpr: TypeParamRef => TypeB.M(tpr.paramNum)
+            case _ => TypeB.None
+          pt.resType match
+            case mt: MethodType =>
+              sym.addAnnotation(ErasedInfo(pt.paramInfos.size, mt.paramInfos.map(toTypeB), toTypeB(mt.resType)))
+            case other =>
+              sym.addAnnotation(ErasedInfo(pt.paramInfos.size, Nil, toTypeB(other.widenExpr)))
+        case _ => ()
+    tree
+}
+
+object ErasurePreservation {
+  val name: String = "erasure preservation"
+  val description: String = "preserve information in annotations before erasure"
+}
+
+enum TypeB:
+  case None
+  case M(x: Int)
+// case class TypeB(tp: Type)
+
+class ErasedInfo(paramCount: Int, paramType: List[TypeB], returnType: TypeB) extends Annotation {
+  override def tree(using Context) =
+    tpd.New(defn.SourceFileAnnot.typeRef,
+            List(tpd.Literal(Constant(toString))))
+
+  override def toString =
+    s"$paramCount, $paramType, $returnType"
+}