Warn on unused import of deprecated element

[som-snytt]

Usually the usage will warn, but for language imports, there is no usage, so you might get an unused import instead.

Similarly, if an import of implicits is no longer necessary, I'd rather see the deprecation than an unused import.

Knock-on to #11753

[inkytonik]

Closely related to this issue is one that I just noticed. I have source that compiles fine with the latest versions of 2.10, 2.11 and 2.12 and with 2.13.0. But when moving to 2.13.1 I get an unused import warning on an import of scala.language.higherKinds. The warning is there because the import is not needed in 2.13.1, except I can't remove it since it's needed for the non-2.13 builds.

Is there any solution to this? For some other version incompatibilities I am adjusting sbt's unmanagedSourceDirectories to pull in appropriate shims so the main code can be the same for all Scala versions. But I don't see how a solution like that is even possible for one of these special language imports. And I it won't be helped by changing it into a deprecation warning.

[joroKr21]

Use -language:higherKinds depending on the version and drop the import.

[inkytonik]

Thanks @joroKr21 . Setting the option in the build is possible, although without further investigation I'm not sure how to set that option just for two files out of my build, since I don't want to turn that flag on for all files.

All of this seems to be a non-trivial flow-on effect of a minor x.x.0 -> x.x.1 update. I would expect changes with this kind of impact to be in major updates.

[som-snytt]

@inkytonik It's messy, as per the ticket.

It just popped into my head:

$ scala -Xlint
Welcome to Scala 2.13.1 (OpenJDK 64-Bit Server VM, Java 1.8.0_222).
Type in expressions for evaluation. Or try :help.

scala> implicitly[scala.languageFeature.higherKinds]
                 ^
       error: could not find implicit value for parameter e: languageFeature.higherKinds

scala> { import language.higherKinds }
                         ^
       warning: Unused import

scala> { import language.higherKinds ; implicitly[scala.languageFeature.higherKinds] }
                                                                        ^
       warning: trait higherKinds in object languageFeature is deprecated (since 2.13.1): scala.language.higherKinds no longer needs to be imported explicitly
                                                 ^
       warning: lazy value higherKinds in object language is deprecated (since 2.13.1): higherKinds no longer needs to be imported explicitly
res2: languageFeature.higherKinds = scala.languageFeature$higherKinds$@4eed2acf

scala> { import language.higherKinds ; @deprecated("","") def f = implicitly[scala.languageFeature.higherKinds] }
                                                              ^
       warning: local method f in value res3 is never used

scala> { import language.higherKinds ; @annotation.unused @deprecated("","") def f = implicitly[scala.languageFeature.higherKinds] }

Edit: the import is also used by def f = higherKinds. Deprecation suppresses the warning for deprecated higherKinds, and marking it unused suppresses that warning.

[som-snytt]

They have @nowarn suppression now.