Scala 3 support

[gianluca-nitti]

Recent scalafix versions support Scala 3. I'm trying to use this plugin in my project which uses Scala 3 and Gradle. Is this possible?

Using release 0.1.13 of the plugin, I get the following behavior:

• With default options (so with semanticdb autoConfigure = true) compilation fails because the compiler is looking for the semanticdb plugin:

  
  > Task :app:compileScala
  [Warn] : bad option '-Yrangepos' was ignored
  No such entry: plugin.properties
  [Warn] : No plugin in path /home/gianluca/.gradle/caches/modules-2/files-2.1/org.scalameta/semanticdb-scalac_2.13.6/4.4.30/87ff27c634c4cecd37f1a41265261f1fd265a71e/semanticdb-scalac_2.13.6-4.4.30.jar
  [Error] : bad option: -P:semanticdb:sourceroot:/home/gianluca/MYPROJECT/app
  two warnings found
  one error found

Task :app:compileScala FAILED

* If I understand correctly, [SemanticDB support is built in the Scala 3 compiler](https://github.com/joan38/mill-scalafix/issues/46#issuecomment-858942933), so there is no plugin which to add. To work around this, I tried disabling the autoConfigure and manually passing the compiler flag needed to enable SemanticDB:
```gradle
// build.gradle
scalafix {
configFile = file("scalafix.conf")
semanticdb {
// this would automatically add compiler flags to generate extra info
// required by some scalafix rules, but is not compatible with Scala 3
// so we add them manually (see below)
autoConfigure = false
}
}

tasks.withType(ScalaCompile) { scalaCompileOptions.additionalParameters = ['-Xsemanticdb'] // for scalafix }

Which however, with semantic rules enabled in `scalafix.conf`, fails with this output:

Task :app:checkScalafixMain FAILED

FAILURE: Build failed with an exception.

• What went wrong: Execution failed for task ':app:checkScalafixMain'.

  The semanticdb compiler plugin is required to run semantic rules such as RemoveUnused. To fix this problem, please enable 'semanticdb.autoConfigure' in the plugin extension

  
  In my understanding, this is happening because `gradle-scalafix` doesn't know SemanticDB is enabled (due to manually enabling the one built-in the compiler). Can this be fixed or worked around somehow?

• Additionally, if I only use syntactic rules in scalafix.conf, I seem to get past the SemanticDB problem but I get parsing errors in files which use certain Scala 3 features such as extension methods. After inspecting the output of gradle check --debug, I think the reason could be that it's using a scalafix artifact for Scala 2.13:

  
  2021-12-10T00:50:24.536+0100 [DEBUG] [io.github.cosmicsilence.scalafix.ScalafixTask] Running Scalafix with the following arguments:

• Mode: CHECK
• Config file: Optional[/home/gianluca/MYPROJECT/app/scalafix.conf]
• Scalafix cli artifact: ch.epfl.scala:scalafix-cli_2.13.7:0.9.33
• External rules classpath:
• Rules: []
• Scala version: 2.13.6
• Scalac options: [-Xsemanticdb]

  
  But I have no idea if and how I can change the scalafix version to use.

[marcelocenerine]

@gianluca-nitti support for Scala 3 in Gradle is relatively recent and this plugin doesn't support that yet 😞 . I don't think there is any workaround you could use atm... The main problem which affects both syntactic and semantic rules is this logic to figure out the version of Scala used in your project, which doesn't correctly work for Scala 3.

[marcelocenerine]

Thanks for bringing this up though. This is something that would be great to add.

Just one note from the Scalafix docs:

Scala 3.x: Scala 3 support is experimental and many built-in rules are not supported.

[gianluca-nitti]

I understand, thanks for the information.

[olafurpg]

[Error] : bad option: -P:semanticdb:sourceroot:/home/gianluca/MYPROJECT/app

The Scala 3 equivalent flag is called -sourceroot and -semanticdb-targetroot for -P:semanticdb:targetroot. One good source of inspiration is the built-in SemanticDB support in sbt https://github.com/sbt/sbt/blob/6c537848266e93674b1de4673c084e8b3f2039df/main/src/main/scala/sbt/plugins/SemanticdbPlugin.scala#L91

I seem to get past the SemanticDB problem but I get parsing errors in files which use certain Scala 3 features such as extension methods.

@marcelocenerine The Scala 3 dialect should be picked up automatically through the --scala-version setting, which seems to be passed correctly here https://github.com/cosmicsilence/gradle-scalafix/blob/master/src/main/groovy/io/github/cosmicsilence/scalafix/ScalafixTask.groovy#L81 cc/ @mlachkar can maybe confirm. Alternatively, it could be that things are working correctly but you're using Scala 3 syntax that's not yet supported by the Scalameta parser (cc/ @tgodzik)

I think the reason could be that it's using a scalafix artifact for Scala 2.13:

Scalafix supports Scala 3 even if Scalafix itself it still compiled with 2.13. There's on Scala 3 artifact for Scalafix yet.

[mlachkar]

In fact Scalafix support scala 3, the only thing we don't have yet is ExcplicitResultType working for scala 3.

• We do configure scala 3 dialect using the scala version (#1392). So if the gradle plugin calls scalafix with the scalaVersion configured, Scalafix will choose the correct dialect. If I remember correctly, we didn't change anything for this part on sbt-scalafix.
• The only change we did on sbt-scalafix, is to configure semanticdb differently for scala3. #210.

@bjaglin can you please confirm ? Thanks

If you need any help, don't hesitate, I would be happy to help!

[marcelocenerine]

Thank you for your prompt response and for sharing all this info, @olafurpg & @mlachkar. I hope get started on this soon 👍