suppressing the warning but not the error

[fommil]

In this code https://github.com/scalaz/ioeffect/blob/6f57538969e38f62f942a05d0b7734483629dc62/src/main/scala/scalaz/ioeffect/Void.scala#L34-L35

we are able to suppress this warning

[error] src/main/scala/scalaz/ioeffect/Void.scala:33:26: dead code following this construct
[error] private[ioeffect] object VoidImpl extends VoidModule with VoidSyntax {
[error]                          ^

but when scalacOptions in Compile += "-Xfatal-warnings" is added, we get

[info] Compiling 13 Scala sources to /tmp/fommil-sbt/home/fommil/Projects/scalaz-ioeffect/target/scala-2.12/classes ...
[error] No warnings can be incurred under -Xfatal-warnings.
[error] one error found

[ghik]

Hello,

It looks like something changed in how -Xfatal-warnings is handled in Scala 2.12.5. This should probably work fine with 2.12.4. I'll dig into it and see if I can make it compatible.

[fommil]

this is scala 2.12.4... we can't upgrade to 2.12.5 because of compiler plugin incompatibilities.

[ghik]

Can't reproduce this with a synthetic test, which means it must be related to SBT and the way it modifies the default scalac reporter. I need to dig more into this.

[ghik]

OK, I think I nailed it.

There is a clash between silencer and semanticdb compiler plugin. Both have their own error/warning reporter which wraps the original reporter. They should totally be able to wrap each other and work together just fine but it's important to ensure that the silencer reporter wraps the semanticdb reporter and not the other way. This means that silencer plugin must be applied after semanticdb plugin, which can be forced using this workaround:

def ensureSilencerIsLast(options: Seq[String]): Seq[String] = {
  val (Seq(silencer), rest) = options.partition(o => o.startsWith("-Xplugin:") && o.contains("silencer"))
  rest :+ silencer
}

scalacOptions in Compile := ensureSilencerIsLast((scalacOptions in Compile).value)
scalacOptions in Test := ensureSilencerIsLast((scalacOptions in Test).value)

[fommil]

@olafurpg @xeno-by FYI...

[olafurpg]

there are similar conflicts with clippy, I'm not sure what's going on because semanticdb forwards all messages to the underlying reporter so silencer should get messages even if semanticdb is enabled last.

[ghik]

@olafurpg semanticdb reporter might properly forward all the messages to its underlying reporter but it still maintains its own error/warning count. This means its warning count will also include warnings suppressed by silencer and when the compiler asks it "are there warnings?" it will answer "yes" even though they were suppressed.

One way to fix this is to synchronize error/warning counts with the underlying reporter. This is what silencer's SuppressingReporter does.

However, I think it would still be better if silencer was the last compiler plugin applied because semanticdb reporter still sees and stores the suppressed warnings. Only by making silencers reporter the toplevel one we can make sure that truly nobody ever sees warnings suppressed by it.

[fommil]

I think it would be good if all plugins tried to be good citizens to the best of their ability, with silencer at the end of the list as the nuclear option.

[olafurpg]

@ghik aah, good catch! I've opened scalameta/scalameta#1505 to track this, I suspect synchronizing info/warn/error count should be a simple fix.

I think it would actually be great to have silenced messages in SemanticDB since those can be useful for tooling. I've always wanted a mode to store in SemanticDB "Unused import" warnings without reporting them in the console since scalafix can automatically fix them.

[ghik]

Ok, I think things are clear now and there isn't anything to do about it in silencer itself, so I'm closing this issue.

[fommil]

thanks to you both!