False positive deprecation warnings when `-Wunused:imports` is used in 3.4.0-RCx

[kyri-petrou]

Compiler version

All of 3.4.0-RCx (no issues with 3.3.x)

Minimized code

Unfortunately I wasn't able to minimize, but I hope I can explain the issue well enough to identify what's causing it

Context / Setup

One of our (large) service at $WORK uses Scala 3 for quite some time now. I wanted to try out 3.4.0-RC4, but came across a very weird regression when Wunused:imports is enabled.

The service uses Thrift clients that are generated via scrooge, and are defined as a dependency via the Scala 2.13 cross CrossVersion.for3Use2_13 flag. The client is initialized as shown below (Note that the reflectiveSelectable import is required so that we can close the client. I also tried removing the reflection, but the issue still persists)

  import reflect.Selectable.reflectiveSelectable

  type SvcType = {
    def asClosable: Closable
  } & ThriftService

  def createServiceClient[S <: SvcType](address: String)(using ClassTag[S]): S =
    Thrift.client.build[S](address)

Issue

Since 3.4.0-RC1, having the following code in the project causes ~3,500 deprecation warnings to be raised, many of them that are not even part of the project (i.e., 3rd-party libraries)

...
[warn] .../src/main/scala-3/io/scalaland/chimney/internal/compiletime/ExprPromisesPlatform.scala:  <none> is deprecated since 2019-10-03: Use thriftReusableBufferFactory instead
[warn] .../src/main/scala-3/zio/ZLayerCompanionVersionSpecific.scala:  <none> is deprecated since 2019-10-03: Use thriftReusableBufferFactory instead
[warn] 3387 warnings found

After searching for the 2019-10-03 string in the finagle codebase, I found this method which contains an argument that is annotated as deprecated.

The (very) weird thing here is that the service codebase does not call this method, at least not directly. Removing the -Wunused:imports compiler flag makes the issue go away. Similarly, using ??? as the body for the createServiceClient method above also makes the issue go away.

Hope this helps, please let me know if you need some more info to help track down what's causing this. Unfortunately, there's no workaround (unless linting is disabled which is not possible)

[Gedochao]

The question here if this is indeed a valid bug, or if the deprecation warning should indeed have been raised, while it wasn't in earlier versions... Hard to say without minimising this.

Tried reproducing around code similar to

//> using scala 3.4.0-RC4
//> using dep com.twitter:finagle-thrift_2.13:23.11.0
//> using options -Wunused:imports
import com.twitter.finagle.thrift.RichClientParam.apply

But it behaves as expected (as it should).
I can't seem to get any weird behaviour with the following, either.

object ThriftMayBehaveFunny {
  def reproAttempt = com.twitter.finagle.thrift.RichClientParam(
    maxThriftBufferSize = 1
  )
}
import ThriftMayBehaveFunny.reproAttempt

The (very) weird thing here is that the service codebase does not call this method, at least not directly. Removing the -Wunused:imports compiler flag makes the issue go away. Similarly, using ??? as the body for the createServiceClient method above also makes the issue go away.

@kyri-petrou we may need something more to go on. Perhaps you'd be able to find the indirect way com.twitter.finagle.thrift.RichClientParam.apply may be called? Or maybe it's some other deprecation entirely?

Bringing in @szymon-rd to the conversation, perhaps there's something obvious here I can't see.

[szymon-rd]

Do you maybe know why it reports warnings in directories like src/main/scala-3/io/scalaland/chimney/internal/? Are you building dependencies from sources?

[kyri-petrou]

This is the closest I can get to a repro / minimization. Note that the zio dependency / zlayer construction is added because the issue doesn't exist without it. It could have been be some other library / macro, but this is the first that came to mind. The issue seems to be present only whenever a macro is involved:

//> using scala 3.4.0-RC4
//> using dep com.twitter:finagle-thrift_2.13:23.11.0
//> using dep dev.zio::zio:2.0.21
//> using options -Wunused:imports -deprecation 

import com.twitter.finagle.*
import com.twitter.finagle.thrift.ThriftService
import scala.reflect.ClassTag
import zio.*

final class Unrelated()

object Unrelated {
    val layer = ZLayer.derive[Unrelated]
}

object Main {
    def foo[S <: ThriftService](using ClassTag[S]) = Thrift.client.build[S]("asd")
}

[kyri-petrou]

Note that in the service I mentioned I also noticed the issue with some inline def methods, but I couldn't figure out how to reproduce it using one of those

[kyri-petrou]

Do you maybe know why it reports warnings in directories like src/main/scala-3/io/scalaland/chimney/internal/? Are you building dependencies from sources?

Nope, doing nothing like this. I hope the reproduction above might shed some light on what's going on because it's really puzzling behaviour

[Gedochao]

Okay... we'll still need to minimise it down to a snippet without the zio and finagle dependencies, but at the very least we have some hints now, thanks @kyri-petrou !
Do let us know if you manage to minimise it further.