Ignore unused-imports for scala.collection.compat

valencik · January 30, 2021, 7:44pm

Problem: It’s difficult to use import scala.collection.compat._ in conjunction with fatal-warnings and -Wunused-imports as the Scala 2.13 code is likely to not use the compat import.

There was a PR created to make scala.collection.compat a special case, but it hasn’t been merged:

I don’t think we can use @nowarn or -Wconf to help unfortunately. Some commentary on exploring that avenue is available available here.

Ultimately it would be nice to construct scalac options such the following code compiled without warnings:

package scala.collection {
  package object compat
}

package here.is.a.test {
  import scala.collection.compat._           // no warning
}

e.g. we are able to ignore the unused warning on a particular import
But, with the same settings have the following code warn on the ListBuffer import:

package scala.collection {
  package object compat
}

package here.is.a.test {
  import scala.collection.compat._           // no warning
  import scala.collection.mutable.ListBuffer // warning
}

This came up in trying to adopt scala-collection-compat in http4s.

sideeffffect · January 30, 2021, 9:11pm

I think I (and many others) had similar issues with scala-parallel-collections. This solution worked for us:

github.com

zio/zio-prelude/blob/e309bb8f0939070e9a0dbbfbcf71f731e22941d6/core/jvm/src/test/scala/zio/prelude/IdentityParIterableSpec.scala#L10


package zio.prelude

import com.github.ghik.silencer.silent
import zio.blocking.Blocking
import zio.prelude.newtypes.Sum
import zio.test._

@silent
object IdentityParIterableSpec extends DefaultRunnableSpec {
  private val ParallelCollectionCompatibility = {
    object Compat {
      object CollectionConverters
    }
    import Compat._
    {
      import scala.collection.parallel._
      CollectionConverters
    }
  }
  import ParallelCollectionCompatibility._

Maybe it (or something like that) will work for you with scala-collection-compat