support plugins on scala_library about rules_scala

[johnynek]

can you give some links here? There are plugins for the scala compiler you mean?

[johnynek]

can't you use scalaopts https://github.com/bazelbuild/rules_scala/blob/master/scala/scala.bzl#L274

and pass -Psomeplugin:opts?

[smparkes]

http://bazel.io/docs/be/java.html#java_plugin
https://github.com/google/auto/tree/master/service

It's actually java annotation processing. And, yeah, scala and java have different concepts of annotation plugins.

In this particular case, I think it'd be possible to add a javac step to make things work though that won't work for other annotation processors / plugins.

I might be able to play with this in a couple of weeks ...

[johnynek]

from what I can see, those have to be applied with javac. I guess if you have the annotation in scope when you compile, scalac won't complain, then you need a step where javac is called on the resulting jar with the plugin and I guess a new jar is produced?

[smparkes]

Okay, I've looked into this a bit more. I created this issue before I really understood how JSR 269 annotation processing works.

It is possible to do this, along the lines of what you mention: the only diff is you have to explode the jar and pass all the classes to javac, along with -proc:only, rather than pass in the intact jar.

But that seems like it's going too far. This isn't a bazel issue, it's a java vs. scala issue. And if we did this, it would only work for this one kind of annotation processing, since arbitrary JSR 269 annotation processors can do things that wouldn't work for this.

I think the right answer is to just create a native java class. In my case, I just extend a scala class, e.g.,

@AutoService(Configurator.class)
public class JavaLogbackConfigurator extends LogbackConfigurator {}

This seems like a perfectly reasonable approach.

It's a little kludgy right now since we have to do the java_import trampoline to get scala libraries into java builds but there's another issue for that ...

[johnynek]

ok. So this adds to the need to solve #18