@nowarn annotation does not suppress any warnings when compiling SBT plugins

[dziemba]

steps

Trying to build an SBT plugin with sbt 1.5.0, in our case twinagle.

problem

We get @nowarn annotation does not suppress any warnings errors in the code that extends AutoPlugin.

expectation

No warnings/errors are emitted.

notes

Seems related to #6398

[adpi2]

Thanks for reporting!

This bug is indeed related to #6398

We will be able to fix it by adding scalacOptions += "-Wconf:cat=unused-nowarn:s" to the SbtPlugin.projectSettings

As a temporary fix you should do the same in your build.

[dziemba]

Thanks for the quick fix!

[dziemba]

Hey @adpi2 and @eed3si9n, it looks like the problem still happens with sbt 1.5.1, at least under certain circumstances. Here's a minimal example:

$ cat build.sbt
sbtPlugin := true
scalacOptions ++= Seq("-Xlint", "-Xfatal-warnings")

$ cat project/build.properties
sbt.version=1.5.1

$ cat src/main/scala/Test.scala
import sbt.Keys._
import sbt._

object Test extends AutoPlugin {
  override def projectSettings: Seq[Setting[_]] = Seq(organization := "foo")
}

$ sbt compile
[info] welcome to sbt 1.5.1 (AdoptOpenJDK Java 1.8.0_282)
[info] loading project definition from ~/tmp/project
[info] loading settings for project tmp from build.sbt ...
[info] set current project to tmp (in build file:~/tmp/)
[info] Executing in batch mode. For better performance use sbt's shell
[info] compiling 1 Scala source to ~/tmp/target/scala-2.12/sbt-1.0/classes ...
[error] ~/tmp/src/main/scala/Test.scala:5:68: @nowarn annotation does not suppress any warnings
[error]   override def projectSettings: Seq[Setting[_]] = Seq(organization := "foo")
[error]                                                                    ^
[error] one error found
[error] (Compile / compileIncremental) Compilation failed

[dziemba]

Ah, I think I figured this out. Looking at the fix (https://github.com/sbt/sbt/pull/6431/files) it only works when using enablePlugins(SbtPlugin) instead of sbtPlugin := true.

I assume that using the plugin is (now) preferred over just setting the sbtPlugin key?

[eed3si9n]

@dziemba Thanks for the detective work. It's true that using SbtPlugin is generally preferred but I am not sure if it's ok to start displaying false warnings without it. A quick fix might be to move the fix into a regular JvmPlugin settings.