Using Twitter Scrooge with Mill

build.sc

import scalalib._
import mill._
import mill.define.Sources
import mill.api.PathRef
import $ivy.`com.twitter::scrooge-generator:19.11.0`

object example extends Cross[ExampleModule]("2.12.8")

class ExampleModule(val crossScalaVersion: String) extends CrossSbtModule {

  // Must match the version from scrooge-generator imported above
  val scroogeVersion = "19.11.0"

  // The structure loosely follows that of https://github.com/lihaoyi/mill/blob/master/contrib/scalapblib/src/ScalaPBModule.scala albeit with a lot of inlining

  override def generatedSources = T { super.generatedSources() :+ compileScrooge() }

  def scroogeSources: Sources = T.sources {
    millSourcePath / 'src / 'main / 'thrift
  }

  def compileScrooge: T[PathRef] = T {
    val dest = T.ctx().dest
    def compileDir(inputDir: os.Path) {
      // ls throws if the path doesn't exist
      if (inputDir.toIO.exists) {
        val thriftPaths = os.walk(inputDir).filter(_.last.matches(".*\\.thrift")).map(_.toIO.getCanonicalPath)
        com.twitter.scrooge.Main.main(Array("-d", dest.toIO.getCanonicalPath, "-l", "scala", "--finagle") ++ thriftPaths)
      }
    }
    scroogeSources().map(_.path).foreach(compileDir)
    mill.api.Result.Success(PathRef(dest))
  }

  def ivyDeps = Agg(
    ivy"com.twitter::scrooge-core:${scroogeVersion}",
    ivy"com.twitter::scrooge-serializer:${scroogeVersion}",
    ivy"com.twitter::finagle-thrift:${scroogeVersion}",
    ivy"org.apache.thrift:libthrift:0.10.0",
  )

}

// Sample directory structure:
//
// example/src/main/thrift/Example.thrift:
//
//   namespace java com.example
//
//   struct Example {
//     1: optional string example
//   }
//
// example/src/main/scala/com/example/Main.scala:
//
//   package com.example
//
//   object Main {
//     def main(args: Array[String]) {
//       println(com.twitter.scrooge.BinaryThriftStructSerializer(Example).toString(Example(Some("mill-scrooge"))))
//     }
//   }
//
// Then running mill will yield
//
//   % mill 'example[2.12.8].run'
//   [41/41] example[2.12.8].run
//   CwABAAAADG1pbGwtc2Nyb29nZQA=