-Ywarn-unused-import triggering on play routes file

I want to be able to use -Xfatal-warnings and -Ywarn-unused-import, the problem is that the compiler is triggering an error on the file which contains the play routes for my application:

[error] /path/to/app/conf/routes: Unused import
[error] /path/to/app/conf/routes: Unused import
[error] /path/to/app/conf/routes:1: Unused import
[error] GET        /document/:id        my.app.controllers.MyController.getById(id: Int)

same goes for other routes.

Is it possible maybe to tell scalac to ignore a file?

Scala version is 2.11.8.

I just encountered the same problem with Scala 2.12 and Play 2.6 (which you're probably now using).

A Scala compiler plugin called Silencer sorts it out: https://github.com/ghik/silencer

Add the following dependency into build.sbt:

val silencerVersion = "1.2.1"

libraryDependencies ++= Seq(
    compilerPlugin("com.github.ghik" %% "silencer-plugin" % silencerVersion),
    "com.github.ghik" %% "silencer-lib" % silencerVersion % Provided
)

Then add (also in build.sbt):

scalacOptions += "-P:silencer:globalFilters=Unused import"

The text after the globalFilters= is a list of regex matches for compiler warnings to silence, can be comma-separated. You might need to tweak the regex for your own case, but I found the above example worked fine.

It does mean that it silences any "Unused import" warnings, but if you're in the habit of auto-formatting your code (ctrl+alt+L in Intellij), including tidying up unused imports, then it shouldn't cause a problem.

A horrible "solution" could be to remove those unused imports after the routes are generated but before the compile task runs. Here's a sketch:

lazy val optimizeRoutesImports = taskKey[Unit]("Remove unused imports from generated routes sources.")

optimizeRoutesImports := {

  def removeUnusedImports(targetFiles: (File) => PathFinder, linesToRemove: Set[String], linesToReplace: Map[String, String]) = {
    val files = targetFiles(crossTarget.value).get
    files foreach { file =>
      val lines = sbt.IO.readLines(file)
      val updatedLines = lines map { line =>
        linesToReplace.getOrElse(line, line)
      } filterNot { line =>
        linesToRemove.contains(line.trim)
      }
      sbt.IO.writeLines(file, updatedLines, append = false)
    }
  }

  removeUnusedImports(
    _ / "routes" / "main" / "controllers" / "ReverseRoutes.scala",
    Set("import ReverseRouteContext.empty"),
    Map(
      "import play.api.mvc.{ QueryStringBindable, PathBindable, Call, JavascriptLiteral }" ->
        "import play.api.mvc.{ QueryStringBindable, PathBindable, Call }",
      "import play.core.routing.{ HandlerDef, ReverseRouteContext, queryString, dynamicString }" ->
        "import play.core.routing.{ ReverseRouteContext, queryString, dynamicString }"
    )
  )

  removeUnusedImports(
    _ / "routes" / "main" / "controllers" / "javascript" / "JavaScriptReverseRoutes.scala",
    Set(
      "import play.core.routing.{ HandlerDef, ReverseRouteContext, queryString, dynamicString }",
      "import ReverseRouteContext.empty"
    ),
    Map(
      "import play.api.mvc.{ QueryStringBindable, PathBindable, Call, JavascriptLiteral }" ->
        "import play.api.mvc.{ QueryStringBindable, PathBindable }"
    )
  )

  removeUnusedImports(
    _ / "routes" / "main" / "router" / "Routes.scala",
    Set("import play.core.j._"),
    Map())
}

You'll then want to sort out the task dependencies:

// Our optimize routes imports task depends on the routes task.
optimizeRoutesImports := (optimizeRoutesImports dependsOn (play.sbt.routes.RoutesKeys.routes in Compile)).value

// And compilation depends on the unused routes having been removed.
compile := ((compile in Compile) dependsOn optimizeRoutesImports).value

You'll also likely need to set TwirlKeys.templateImports to a conservative list before enabling -Ywarn-unused-import. Something like this, depending on what types are used in your views:

TwirlKeys.templateImports := Seq("play.api.mvc._", "play.api.i18n.Messages", "controllers.routes")

I also had to knock unused TemplateMagic imports out of Twirl templates (YMMV):

  removeUnusedImports(
    _ / "twirl" ** "*.template.scala",
    Set("import play.twirl.api.TemplateMagic._"),
    Map())

If you do that, make sure the task dependencies are set up appropriately.

This works for me. Scala 2.11.8, Play 2.5.10, both -Xfatal-warnings and -Ywarn-unused-import enabled. It's hideous, but it works.

I've come up with a working solution for Scala 2.13.7 (no need for any plugin) for Play 2.8.11. Take a look on those examples and adjust to your needs:

scalacOptions ++= Seq(
    "-Wconf:cat=unused-imports&site=.*views.html.*:s", // Silence import warnings in Play html files
    "-Wconf:cat=unused-imports&site=<empty>:s", // Silence import warnings on Play `routes` files
    "-Wconf:cat=unused-imports&site=router:s", // Silence import warnings on Play `routes` files
    "-Wconf:cat=unused-imports&site=v1:s", // Silence import warnings on Play `v1.routes` files
    "-Wconf:cat=unused-imports&site=v2:s", // Silence import warnings on Play `v2.routes` files
    "-Wconf:cat=unused-imports&site=views.v1:s", // Silence import warnings on Play `views.v1.routes` files
    "-Wconf:cat=deprecation&site=controllers\\.v1.*&origin=scala.util.Either.right:s", // Silence deprecations in generated Controller classes
    "-Wconf:cat=deprecation&site=.*v1.Routes.*&origin=scala.util.Either.right:s"
  ) // Silence deprecations in generated Controller classes

If you want to understand more, take a look on this documentation and add verbose information on the compiler messages output

scalacOptions += "-Wconf:any:wv",

Pro tip: fail compilation of unused only within CI

scalacOptions ++= {
  // Mark unused errors as info for local development (due to -Werror usage)
  if (insideCI.value) Seq.empty else Seq("-Wconf:cat=unused:i")
},