Split up codegen library

[rafikk]

I would like to use the Twinagle code generator in a Scala project without SBT. In my case, I'll be integrating it with Bazel using a custom scala_proto toolchain from rules_scala.

The code generator is only available as an SBT compiler plugin which presents two problems (1) the library is pegged at Scala 2.12, and (2) it cannot be used without SBT. So, for example, I cannot and do not wish to depend on the BuildInfo plugin.

The approach taken by this PR is to introduce a new Scala library twinagle-codegen that includes the code generator without any dependencies on SBT and adds this new library as a dependency to the existing twinagle-scalapb-plugin library. I believe this should be transparent to users since twinagle-codegen will be pulled in as a transient dependency.

[BenjaminDebeerst]

For all I understand, this seems like a good change and should indeed be transparent to clients. Looking forward to seeing the sbt-less build :-)

[BenjaminDebeerst]

Maybe the naming could be simplified, since the package location already defines much of the semantics.

I'd suggest to name/move this to com.soundcloud.twinagle.CodeGenerator. If it's a class instead of a trait it could be used without the need of an special Standalone* object.

Consequently, the sbt extension could then be moved to com.soundcloud.twinagle.plugin.CodeGenerator.

[rafikk]

The CodeGenerator defines the main method though (via inheriting the CodeGenApp trait). Doesn't it need to be on a Scala object for it to be statically callable?

[ccmtaylor]

does the trait need to extend CodeGenApp? I wonder if you could have something along the lines of

trait ServerClientCodeGenerator {
  // ...lots of definitions
}

object StandaloneCodeGen extends CodeGenApp with ServerClientCodeGenerator {}
object SbtCodeGen extends CodeGenApp with ServerClientCodeGenerator {
  override def suggestedDependencies = ...
}

[rafikk]

It doesn't work unfortunately, because I need the override def for registerExtensions. When I try that, I get:

[error] object StandaloneServerClientCodeGenerator inherits conflicting members:
[error]   method registerExtensions in trait CodeGenApp of type (registry: com.google.protobuf.ExtensionRegistry)Unit  an
[error]   method registerExtensions in trait ServerClientCodeGenerator of type (registry: com.google.protobuf.ExtensionRegistry)Unit
[error] (Note: this can be resolved by declaring an override in object StandaloneServerClientCodeGenerator.)
[error] object StandaloneServerClientCodeGenerator extends CodeGenApp with ServerClientCodeGenerator {}
[error]        ^
[error] one error found

[ccmtaylor]

nice! this is something I'd considered a long time, since scalapb supports being run as a protoc extension but the integration with sbt made using that impossible 🚀

[rbscgh]

🔥