New anonymous class definition will be duplicated at each inline site

[goshacodes]

Compiler version

3.3.5

Minimized code

trait JsonWriter[T] {
   def writeValues(value: T, out: TokenWriter): Unit
}

object JsonWriter:
  inline def derived[T]: JsonWriter[T] = new JsonWriter[T]:
    def writeValues(value: T, out: TokenWriter): Unit = () // here should be some macro code
}

Output

Warning

New anonymous class definition will be duplicated at each inline site

Expectation

No warning. Disabling warning in all project may cause that I miss correct warnings

Why I expect no warning?

1. Because for different types generated body may vary for performance reasons. (Not full example, just to explain)

case class Foo(foo: String)

new JsonWriter[Foo]:
  def writeValues(value: Foo, out: TokenWriter): Unit =
    tokenWriter.writeFieldName("foo")
    tokenWriter.writeString(value.foo)
  

case class Bar(bar: Int)

new JsonWriter[Bar]:
  def writeValues(value: Bar, out: TokenWriter): Unit =
    tokenWriter.writeFieldName("bar")
    tokenWriter.writeint(value.bar)

2. Creating class now allows to pass some compile time configuration to derivation

sealed trait WriterBuilder[A]:
  def rename[B](field: A => B)(rename: String): WriterBuilder[A]
  def fieldStyle(style: FieldStyle): WriterBuilder[A]

object WriterBuilder:
  @scala.annotation.compileTimeOnly(
    "Should be declared as inline given or provided directly to derives"
  )
  def apply[A](using
      mirror: scala.deriving.Mirror.ProductOf[A]
  ): WriterBuilder[A] =
    throw IllegalStateException(
      "Config must be an inlined given or provided directly to 'derived'"
    )

Usage is:

given JsonWriter[Foo] = JsonWriter.derived {
  WriterBuilder[Foo].rename(_.foo)("baz") 
}

// and now it should generate

new JsonWriter[Foo]:
  def writeValues(value: Foo, out: TokenWriter): Unit =
    tokenWriter.writeFieldName("baz")
    tokenWriter.writeString(value.foo)
  

[som-snytt]

This is what @nowarn is for. This is a warning at definition and not at expansion. (The latter works with nowarn now.)

[goshacodes]

This means users won't see it even if I leave it without @nowarn?

This is what @nowarn is for. This is a warning at definition and not at expansion

[bishabosha]

also, if you want to specialize then its better to create those specialized examples at definition site, rather than every user getting a new class defnitions - the reason this warning was created was because it caused significant code cache bloating in jvm for larger apps like lichess.org when they first migrated to Scala 3