Discover main class(es) via zinc

3 files changed, 42 insertions, 5 deletions

diff --git a/scalalib/src/mill/scalalib/ScalaModule.scala b/scalalib/src/mill/scalalib/ScalaModule.scala
index b7810246..f0b2ca86 100644
--- a/scalalib/src/mill/scalalib/ScalaModule.scala
+++ b/scalalib/src/mill/scalalib/ScalaModule.scala
@@ -23,7 +23,19 @@ trait ScalaModule extends mill.Module with TaskModule { outer =>
     override def moduleDeps = Seq(outer)
   }
   def scalaVersion: T[String]
-  def mainClass: T[Option[String]] = None
+
+  def mainClass: T[Option[String]] = T{
+    discoverMainClasses() match {
+      case Seq(main) => Some(main)
+      case _         => None
+    }
+  }
+
+  def discoverMainClasses: T[Seq[String]] = T{
+    Task.traverse(transitiveModuleDeps){ module => T.task {
+      mill.scalalib.ScalaWorkerApi.scalaWorker().discoverMainClasses(module.compile())
+    }}().flatten.distinct
+  }
 
   def ivyDeps = T{ Agg.empty[Dep] }
   def compileIvyDeps = T{ Agg.empty[Dep] }
diff --git a/scalalib/src/mill/scalalib/ScalaWorkerApi.scala b/scalalib/src/mill/scalalib/ScalaWorkerApi.scala
index 03cd7d9d..ec2489af 100644
--- a/scalalib/src/mill/scalalib/ScalaWorkerApi.scala
+++ b/scalalib/src/mill/scalalib/ScalaWorkerApi.scala
@@ -62,4 +62,6 @@ trait ScalaWorkerApi {
                testClassfilePath: Agg[Path],
                args: Seq[String])
               (implicit ctx: mill.util.Ctx.Log): (String, Seq[Result])
+
+  def discoverMainClasses(compilationResult: CompilationResult)(implicit ctx: mill.util.Ctx): Seq[String]
 }
diff --git a/scalalib/test/src/mill/scalalib/HelloWorldTests.scala b/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
index 1d39f180..945bec27 100644
--- a/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
+++ b/scalalib/test/src/mill/scalalib/HelloWorldTests.scala
@@ -33,6 +33,15 @@ object HelloWorldTests extends TestSuite {
     class HelloWorldCross(val crossScalaVersion: String) extends CrossScalaModule
   }
 
+  object HelloWorldDefaultMain extends HelloBase {
+    object core extends HelloWorldModule
+  }
+
+  object HelloWorldWithoutMain extends HelloBase {
+    object core extends HelloWorldModule{
+      def mainClass = None
+    }
+  }
 
   object HelloWorldWithMain extends HelloBase {
     object core extends HelloWorldModule{
@@ -291,8 +300,8 @@ object HelloWorldTests extends TestSuite {
           read(runResult) == "hello rockjam, your age is: 25"
         )
       }
-      'notRunWithoutMainClass - workspaceTest(HelloWorld){eval =>
-        val Left(Result.Exception(err, _)) = eval.apply(HelloWorld.core.run())
+      'notRunWithoutMainClass - workspaceTest(HelloWorldWithoutMain){eval =>
+        val Left(Result.Exception(err, _)) = eval.apply(HelloWorldWithoutMain.core.run())
 
         assert(
           err.isInstanceOf[RuntimeException]
@@ -314,8 +323,22 @@ object HelloWorldTests extends TestSuite {
           read(runResult) == "hello rockjam, your age is: 25"
         )
       }
-      'notRunWithoutMainClass - workspaceTest(HelloWorld){eval =>
-        val Left(Result.Exception(err, _)) = eval.apply(HelloWorld.core.runLocal())
+      'runWithDefaultMain - workspaceTest(HelloWorldDefaultMain){eval =>
+        val runResult = eval.outPath / 'core / 'run / 'dest / "hello-mill"
+        val Right((_, evalCount)) = eval.apply(
+          HelloWorldDefaultMain.core.runLocal(runResult.toString)
+        )
+
+        assert(evalCount > 0)
+
+
+        assert(
+          exists(runResult),
+          read(runResult) == "hello rockjam, your age is: 25"
+        )
+      }
+      'notRunWithoutMainClass - workspaceTest(HelloWorldWithoutMain){eval =>
+        val Left(Result.Exception(err, _)) = eval.apply(HelloWorldWithoutMain.core.runLocal())
 
         assert(
           err.isInstanceOf[RuntimeException]