Work around "a pure expression does nothing" warning, take 3

[eed3si9n]

Fixes #6161
Ref scala/bug#12112

This uses the configurable warning backported to Scala 2.12.

[adpi2]

@eed3si9n This PR breaks the dotty repository. I get 100 errors like these:

[info] compiling 10 Scala sources and 1 Java source to /home/piquerez/lampepfl/dotty/project/target/scala-2.12/sbt-1.0/classes ...
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1375:23: value annotation is not a member of sbt.InputKey[Unit]
[error]     publishMavenStyle := true,
[error]                       ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1376:16: value annotation is not a member of sbt.InputKey[Unit]
[error]     isSnapshot := version.value.contains("SNAPSHOT"),
[error]                ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1377:15: value annotation is not a member of sbt.InputKey[Unit]
[error]     publishTo := sonatypePublishToBundle.value,
[error]               ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1380:29: value annotation is not a member of sbt.InputKey[Unit]
[error]     publishArtifact in Test := false,
[error]                             ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1381:14: value annotation is not a member of sbt.InputKey[Unit]
[error]     homepage := Some(url(dottyGithubUrl)),
[error]              ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1382:14: value annotation is not a member of sbt.InputKey[Unit]
[error]     licenses += (("Apache-2.0",
[error]              ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1384:13: value annotation is not a member of sbt.InputKey[Unit]
[error]     scmInfo := Some(
[error]             ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:1390:16: value annotation is not a member of sbt.InputKey[Unit]
[error]     developers := List(
[error]                ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:230:37: value annotation is not a member of sbt.InputKey[Unit]
[error]     scalaSource       in Compile    := baseDirectory.value / "src",
[error]                                     ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:231:37: value annotation is not a member of sbt.InputKey[Unit]
[error]     scalaSource       in Test       := baseDirectory.value / "test",
[error]                                     ^
[error] /home/piquerez/lampepfl/dotty/project/Build.scala:232:37: value annotation is not a member of sbt.InputKey[Unit]
[error]     javaSource        in Compile    := baseDirectory.value / "src"

[eed3si9n]

Sorry! I knew I should've used fqn. Do you have a term named scala somewhere in the file?

[adpi2]

Yes I do:

val scala = inputKey[Unit]("run compiled binary using the correct classpath, or the user supplied classpath")

[adpi2]

Ah, I see the problem now :)

[marcospereira]

Will this be backported to 1.4?

[eed3si9n]

@marcospereira Probably not. "a pure expression does nothing" is at best a usability annoyance, and "take 3" indicates that fixing is not straightforward because it's macro related. 1.5.0-RC1 would be a good candidate for this since it would go through RC cycle, and I'd feel less bad if plugin interaction broke something in RC1.

[andrew-selvia]

In case anyone else arrives here while attempting to decipher why this warning is appearing when they compile their plugin, you may want to bump the sbtVersion within your plugin's build to the minimum version with the fix (1.5.0):

pluginCrossBuild / sbtVersion := { scalaBinaryVersion.value match { case "2.12" => "1.5.0" } },

So long as you're comfortable requiring client projects to use 1.5.0 or later, this will make the warning disappear. Thanks @eed3si9n for all you do!