Add runtime.TupleMirror to avoid anonymous classes for mirrors of tuples

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

@@ -915,6 +915,8 @@ class Definitions {
 
     def TupleXXL_fromIterator(using Context): Symbol = TupleXXLModule.requiredMethod("fromIterator")
 
+  @tu lazy val RuntimeTupleMirrorTypeRef: TypeRef = requiredClassRef("scala.runtime.TupleMirror")
+
   @tu lazy val RuntimeTuplesModule: Symbol = requiredModule("scala.runtime.Tuples")
   @tu lazy val RuntimeTuplesModuleClass: Symbol = RuntimeTuplesModule.moduleClass
     lazy val RuntimeTuples_consIterator: Symbol = RuntimeTuplesModule.requiredMethod("consIterator")

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

@@ -26,12 +26,6 @@ object SyntheticMembers {
 
   /** Attachment recording that an anonymous class should extend Mirror.Sum */
   val ExtendsSumMirror: Property.StickyKey[Unit] = new Property.StickyKey
-
-  /** Attachment recording that an anonymous class (with the ExtendsProductMirror attachment)
-   *  should implement its `fromProduct` method in terms of the runtime class corresponding
-   *  to a tuple with that arity.
-   */
-  val GenericTupleArity: Property.StickyKey[Int] = new Property.StickyKey
 }
 
 /** Synthetic method implementations for case classes, case objects,
@@ -607,11 +601,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
     else if (impl.removeAttachment(ExtendsSingletonMirror).isDefined)
       makeSingletonMirror()
     else if (impl.removeAttachment(ExtendsProductMirror).isDefined)
-      val tupleArity = impl.removeAttachment(GenericTupleArity)
-      val cls = tupleArity match
-        case Some(n) => defn.TupleType(n).nn.classSymbol
-        case _ => monoType.typeRef.dealias.classSymbol
-      makeProductMirror(cls)
+      makeProductMirror(monoType.typeRef.dealias.classSymbol)
     else if (impl.removeAttachment(ExtendsSumMirror).isDefined)
       makeSumMirror(monoType.typeRef.dealias.classSymbol)
 

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -223,19 +223,16 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
   /** Create an anonymous class `new Object { type MirroredMonoType = ... }`
    *  and mark it with given attachment so that it is made into a mirror at PostTyper.
    */
-  private def anonymousMirror(monoType: Type, attachment: Property.StickyKey[Unit], tupleArity: Option[Int], span: Span)(using Context) =
+  private def anonymousMirror(monoType: Type, attachment: Property.StickyKey[Unit], span: Span)(using Context) =
     if ctx.isAfterTyper then ctx.compilationUnit.needsMirrorSupport = true
     val monoTypeDef = untpd.TypeDef(tpnme.MirroredMonoType, untpd.TypeTree(monoType))
-    var newImpl = untpd.Template(
+    val newImpl = untpd.Template(
       constr = untpd.emptyConstructor,
       parents = untpd.TypeTree(defn.ObjectType) :: Nil,
       derived = Nil,
       self = EmptyValDef,
       body = monoTypeDef :: Nil
     ).withAttachment(attachment, ())
-    tupleArity.foreach { n =>
-      newImpl = newImpl.withAttachment(GenericTupleArity, n)
-    }
     typer.typed(untpd.New(newImpl).withSpan(span))
 
   /** The mirror type
@@ -385,12 +382,17 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
 
   private def productMirror(mirroredType: Type, formal: Type, span: Span)(using Context): TreeWithErrors =
 
+    /** `new scala.runtime.TupleMirror(arity)`
+     *  using TupleMirror avoids generating anonymous classes for tuple mirrors.
+     */
+    def newTupleMirror(arity: Int): Tree =
+      New(defn.RuntimeTupleMirrorTypeRef, Literal(Constant(arity)) :: Nil)
+
     def makeProductMirror(cls: Symbol, tps: Option[List[Type]]): TreeWithErrors =
       val accessors = cls.caseAccessors.filterNot(_.isAllOf(PrivateLocal))
       val elemLabels = accessors.map(acc => ConstantType(Constant(acc.name.toString)))
-      val nestedPairs =
-        val elems = tps.getOrElse(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))
-        TypeOps.nestedPairs(elems)
+      val typeElems = tps.getOrElse(accessors.map(mirroredType.resultType.memberInfo(_).widenExpr))
+      val nestedPairs = TypeOps.nestedPairs(typeElems)
       val (monoType, elemsType) = mirroredType match
         case mirroredType: HKTypeLambda =>
           (mkMirroredMonoType(mirroredType), mirroredType.derivedLambdaType(resType = nestedPairs))
@@ -406,7 +408,8 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
       }
       val mirrorRef =
         if cls.useCompanionAsProductMirror then companionPath(mirroredType, span)
-        else anonymousMirror(monoType, ExtendsProductMirror, tps.map(_.size), span)
+        else if defn.isTupleClass(cls) then newTupleMirror(typeElems.size) // TODO: cls == defn.PairClass when > 22
+        else anonymousMirror(monoType, ExtendsProductMirror, span)
       withNoErrors(mirrorRef.cast(mirrorType))
     end makeProductMirror
 
@@ -508,7 +511,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
         }
       val mirrorRef =
         if cls.useCompanionAsSumMirror then companionPath(mirroredType, span)
-        else anonymousMirror(monoType, ExtendsSumMirror, None, span)
+        else anonymousMirror(monoType, ExtendsSumMirror, span)
       withNoErrors(mirrorRef.cast(mirrorType))
     else if acceptableMsg.nonEmpty then
       withErrors(i"type `$mirroredType` is not a generic sum because $acceptableMsg")

library/src/scala/runtime/TupleMirror.scala

@@ -0,0 +1,14 @@
+package scala.runtime
+
+/** A concrete subclass of `scala.deriving.Mirror.Product`, enabling reduction of bytecode size.
+ *  as we do not need to synthesize an anonymous Mirror class at every callsite.
+ */
+final class TupleMirror(arity: Int) extends scala.deriving.Mirror.Product with Serializable:
+  assert(arity >= 0) // technically could be used for EmptyTuple also, but it has its own singleton mirror.
+
+  override type MirroredMonoType <: Tuple
+
+  final def fromProduct(product: Product): MirroredMonoType =
+    if product.productArity != arity then
+      throw IllegalArgumentException(s"expected Product with $arity elements, got ${product.productArity}")
+    runtime.Tuples.fromProduct(product).asInstanceOf[MirroredMonoType]

tests/run/i15399.check

@@ -0,0 +1,4 @@
+Expected failure when pass Tuple3 to TupleMirror(2):
+-  expected Product with 2 elements, got 3
+Expected failure when pass Tuple1 to TupleMirror(2):
+-  expected Product with 2 elements, got 1

tests/run/i15399.scala

@@ -0,0 +1,15 @@
+@main def Test =
+  val tup2Mirror = summon[scala.deriving.Mirror.Of[(Int, Int)]]
+  try
+    val tup2a: (Int, Int) = tup2Mirror.fromProduct((1, 2, 3)) // fails silently and creates (1, 2)
+    println(tup2a)  // should be unreachable
+  catch case err: IllegalArgumentException =>
+    println("Expected failure when pass Tuple3 to TupleMirror(2):")
+    println(s"-  ${err.getMessage}")
+
+  try
+    val tup2b: (Int, Int) = tup2Mirror.fromProduct(Tuple(1)) // crashes with index out of bounds
+    println(tup2b) // should be unreachable
+  catch case err: IllegalArgumentException =>
+    println("Expected failure when pass Tuple1 to TupleMirror(2):")
+    println(s"-  ${err.getMessage}")