Add warning for anonymous inline classes (#16723)

compiler/src/dotty/tools/dotc/printing/Formatting.scala

@@ -42,7 +42,9 @@ object Formatting {
     trait CtxShow:
       def run(using Context): Shown
 
-    private inline def CtxShow(inline x: Context ?=> Shown) = new CtxShow { def run(using Context) = x(using ctx) }
+    private inline def CtxShow(inline x: Context ?=> Shown) =
+      class InlinedCtxShow extends CtxShow { def run(using Context) = x(using ctx) }
+      new InlinedCtxShow
     private def toStr[A: Show](x: A)(using Context): String = Shown.toStr(toShown(x))
     private def toShown[A: Show](x: A)(using Context): Shown = Show[A].show(x).runCtxShow
 

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -208,6 +208,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnstableInlineAccessorID // errorNumber: 192
   case VolatileOnValID // errorNumber: 193
   case ExtensionNullifiedByMemberID // errorNumber: 194
+  case InlinedAnonClassWarningID // errorNumber: 195
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3104,6 +3104,15 @@ extends SyntaxMsg(InlineGivenShouldNotBeFunctionID):
        |      inline def apply(x: A) = x.toB
      """
 
+class InlinedAnonClassWarning()(using Context)
+  extends Message(InlinedAnonClassWarningID):
+    def kind = MessageKind.PotentialIssue
+    def msg(using Context) = "New anonymous class definition will be duplicated at each inline site"
+    def explain(using Context) =
+      i"""Anonymous class will be defined at each use site, which may lead to a larger number of classfiles.
+      |
+      |To inline class definitions, you may provide an explicit class name to avoid this warning."""
+
 class ValueDiscarding(tp: Type)(using Context)
   extends Message(ValueDiscardingID):
     def kind = MessageKind.PotentialIssue

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

@@ -10,6 +10,7 @@ import Symbols.*
 import typer.RefChecks
 import MegaPhase.MiniPhase
 import ast.tpd
+import reporting.InlinedAnonClassWarning
 
 import config.Feature
 import Decorators.*
@@ -51,6 +52,7 @@ class PruneErasedDefs extends MiniPhase with SymTransformer { thisTransform =>
     else cpy.ValDef(tree)(rhs = trivialErasedTree(tree.rhs))
 
   override def transformDefDef(tree: DefDef)(using Context): Tree =
+    RefChecks.checkNoInlineAnnoClasses(tree)
     checkErasedInExperimental(tree.symbol)
     if !tree.symbol.isEffectivelyErased || tree.rhs.isEmpty then tree
     else cpy.DefDef(tree)(rhs = trivialErasedTree(tree.rhs))

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

@@ -33,6 +33,16 @@ object RefChecks {
     def isStable = true
   }
 
+  def checkNoInlineAnnoClasses(tree: DefDef)(using Context): Unit =
+    if tree.symbol.is(Inline) then
+      new TreeTraverser {
+        def traverse(tree: Tree)(using Context): Unit =
+          tree match
+            case tree: TypeDef if tree.symbol.isAnonymousClass =>
+              report.warning(new InlinedAnonClassWarning(), tree.symbol.sourcePos)
+            case _ => traverseChildren(tree)
+      }.traverse(tree)
+
   /** Only one overloaded alternative is allowed to define default arguments */
   private def checkOverloadedRestrictions(clazz: Symbol)(using Context): Unit = {
     // Using the default getters (such as methodName$default$1) as a cheap way of

tests/neg/i13044.check

@@ -1,3 +1,15 @@
+-- [E195] Potential Issue Warning: tests/neg/i13044.scala:26:8 ---------------------------------------------------------
+26 |        new Schema[A] {
+   |        ^
+   |        New anonymous class definition will be duplicated at each inline site
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E195] Potential Issue Warning: tests/neg/i13044.scala:32:8 ---------------------------------------------------------
+32 |        new Schema[A] {
+   |        ^
+   |        New anonymous class definition will be duplicated at each inline site
+   |
+   | longer explanation available when compiling with `-explain`
 -- Error: tests/neg/i13044.scala:65:40 ---------------------------------------------------------------------------------
 65 |   implicit def typeSchema: Schema[A] = Schema.gen // error // error
    |                                        ^^^^^^^^^^

tests/pos/i17314.scala

@@ -13,8 +13,9 @@ object circelike {
     inline final def derived[A](using conf: Configuration)(using
       inline mirror: Mirror.Of[A]
     ): ConfiguredCodec[A] =
-      new ConfiguredCodec[A]:
+      class InlinedConfiguredCodec extends ConfiguredCodec[A]:
         val codec = summonInline[Codec[URI]] // simplification
+      new InlinedConfiguredCodec
 }
 
 object foo {

tests/pos/not-looping-implicit.scala

@@ -24,10 +24,10 @@ object Schema {
     inline summonInline[Mirror.Of[A]] match {
       case m: Mirror.SumOf[A]     =>
         lazy val members     = recurse[m.MirroredElemLabels, m.MirroredElemTypes]()
-        new Schema[A] {}
+        ???
       case m: Mirror.ProductOf[A] =>
         lazy val fields           = recurse[m.MirroredElemLabels, m.MirroredElemTypes]()
-        new Schema[A] {}
+        ???
     }
 
   inline given gen[A]: Schema[A] = derived[A]

tests/run/i11050.scala

@@ -113,12 +113,14 @@ object Show:
 
   inline def show[T](x: T): String = summonInline[Show[T]].show(x)
 
-  transparent inline def derived[T](implicit ev: Mirror.Of[T]): Show[T] = new {
-    def show(x: T): String = inline ev match {
-      case m: Mirror.ProductOf[T] => showProduct(x.asInstanceOf[Product], m)
-      case m: Mirror.SumOf[T]     => showCases[m.MirroredElemTypes](0)(x, m.ordinal(x))
+  transparent inline def derived[T](implicit ev: Mirror.Of[T]): Show[T] =
+    class InlinedShow extends Show[T] { // provide name to anonymous class
+      def show(x: T): String = inline ev match {
+        case m: Mirror.ProductOf[T] => showProduct(x.asInstanceOf[Product], m)
+        case m: Mirror.SumOf[T]     => showCases[m.MirroredElemTypes](0)(x, m.ordinal(x))
+      }
     }
-  }
+    new InlinedShow
 
   transparent inline def showProduct[T](x: Product, m: Mirror.ProductOf[T]): String =
     constValue[m.MirroredLabel] + showElems[m.MirroredElemTypes, m.MirroredElemLabels](0, Nil)(x)

tests/warn/i15503i.scala

@@ -247,7 +247,7 @@ package foo.test.i16679a:
       import scala.deriving.Mirror
       object CaseClassByStringName:
         inline final def derived[A](using inline A: Mirror.Of[A]): CaseClassByStringName[A] =
-          new CaseClassByStringName[A]:
+          new CaseClassByStringName[A]: // warn
             def name: String = A.toString
 
   object secondPackage:
@@ -263,7 +263,7 @@ package foo.test.i16679b:
     object CaseClassName:
       import scala.deriving.Mirror
       inline final def derived[A](using inline A: Mirror.Of[A]): CaseClassName[A] =
-        new CaseClassName[A]:
+        new CaseClassName[A]: // warn
           def name: String = A.toString
 
   object Foo:
@@ -279,7 +279,7 @@ package foo.test.i17156:
   package a:
     trait Foo[A]
     object Foo:
-      inline def derived[T]: Foo[T] = new Foo{}
+      inline def derived[T]: Foo[T] = new Foo{} // warn
 
   package b:
     import a.Foo

tests/warn/i15503j.scala

@@ -49,11 +49,11 @@ package foo.unused.summon.inlines:
 
     transparent inline given conflictInside: C =
       summonInline[A]
-      new {}
+      ???
 
     transparent inline given potentialConflict: C =
       summonInline[B]
-      new {}
+      ???
 
     val b: B = summon[B]
     val c: C = summon[C]

tests/warn/i16723.check

@@ -0,0 +1,6 @@
+-- [E195] Potential Issue Warning: tests/warn/i16723.scala:3:2 ---------------------------------------------------------
+3 |  new Object {} // warn
+  |  ^
+  |  New anonymous class definition will be duplicated at each inline site
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i16723.scala

@@ -0,0 +1,3 @@
+inline def foo =
+  class NotAnon
+  new Object {} // warn

tests/warn/i16723a.check

@@ -0,0 +1,6 @@
+-- [E195] Potential Issue Warning: tests/warn/i16723a.scala:5:38 -------------------------------------------------------
+5 |inline given Converter[Int, String] = new Converter { // warn
+  |                                      ^
+  |                                      New anonymous class definition will be duplicated at each inline site
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i16723a.scala

@@ -0,0 +1,17 @@
+trait Converter[A, B] {
+  def convert: A => B
+}
+
+inline given Converter[Int, String] = new Converter { // warn
+  def convert = _.toString()
+}
+
+def foo(using bar: Converter[Int, String]) =
+  "foo"
+
+@main
+def main =
+  foo
+  foo
+  foo
+  foo