Add selective.resolveChanged to further help debugging selective test execution

example/fundamentals/out-dir/1-out-files/build.mill

@@ -243,8 +243,7 @@ out/mill-server
 
 > cat out/mill-build/methodCodeHashSignatures.dest/current/spanningInvalidationTree.json
 {
-  "call scala.runtime.BoxesRunTime.boxToInteger(int)java.lang.Integer": {},
-  "call scala.Predef$#println(java.lang.Object)void": {
+...
     "def build_.package_$foo$#<init>(build_.package_)void": {
       "call build_.package_$foo$!<init>(build_.package_)void": {
         "def build_.package_#foo$lzycompute$1()void": {
@@ -254,7 +253,7 @@ out/mill-server
         }
       }
     }
-  }
+...
 }
 
 */

example/large/selective/9-selective-execution/build.mill

@@ -135,13 +135,18 @@ Test run bar.BarTests finished: 0 failed, 0 ignored, 1 total, ...
 // ```
 //
 // For a more detailed report of how the changed inputs resulted in the selected tasks
-// being chosen, you can also use `selective.resolveTree` to print out the selected tasks
+// being chosen, you can also use `selective.resolveChanged` to print out the upstream
+// input tasks that Mill found had changed and will cause downstream tasks to be selected,
+// or `selective.resolveTree` to print out the selected tasks
 // as a JSON tree illustrating the relationships between the invalidated inputs (at the root
 // of the tree) and the selected tasks (at the leaves of the tree)
 //
 
 /** Usage
 
+> mill selective.resolveChanged __.test
+bar.sources
+
 > mill selective.resolveTree __.test
 {
   "bar.sources": {
@@ -176,6 +181,8 @@ Test run bar.BarTests finished: 0 failed, 0 ignored, 1 total, ...
   }
 }
 
+
+
 */
 
 // Similarly, if we make a change `qux/`, using selective execution will only run tests

example/package.mill

@@ -212,7 +212,7 @@ object `package` extends RootModule with Module {
   trait ExampleCrossModule extends build.integration.IntegrationTestModule {
     // disable scalafix because these example modules don't have sources causing it to misbehave
     def testRepoSourceRoot: T[PathRef] = Task.Source(millSourcePath)
-    def testRepoRoot: T[PathRef] = Task{ testRepoSourceRoot() }
+    def testRepoRoot: T[PathRef] = Task { testRepoSourceRoot() }
 
     def sources0 = Task.Sources(millSourcePath)
     def sources = Task {

main/src/mill/main/SelectiveExecution.scala

@@ -113,7 +113,7 @@ private[mill] object SelectiveExecution {
 
   case class ChangedTasks(
       resolved: Seq[NamedTask[_]],
-      changedRootTasks: Set[Task[_]],
+      changedRootTasks: Set[NamedTask[_]],
       downstreamTasks: Seq[NamedTask[_]],
       results: Map[Task[_], Evaluator.TaskResult[Val]]
   )
@@ -142,7 +142,7 @@ private[mill] object SelectiveExecution {
 
       ChangedTasks(
         tasks,
-        changedRootTasks,
+        changedRootTasks.collect { case n: NamedTask[_] => n },
         downstreamTasks.collect { case n: NamedTask[_] => n },
         results
       )
@@ -160,6 +160,12 @@ private[mill] object SelectiveExecution {
     }
   }
 
+  def resolveChanged(evaluator: Evaluator, tasks: Seq[String]): Either[String, Seq[String]] = {
+    for (changedTasks <- SelectiveExecution.computeChangedTasks(evaluator, tasks)) yield {
+      changedTasks.changedRootTasks.map(_.ctx.segments.render).toSeq.sorted
+    }
+  }
+
   def resolveTree(evaluator: Evaluator, tasks: Seq[String]): Either[String, ujson.Value] = {
     for (changedTasks <- SelectiveExecution.computeChangedTasks(evaluator, tasks)) yield {
       val taskSet = changedTasks.downstreamTasks.toSet[Task[_]]

main/src/mill/main/SelectiveExecutionModule.scala

@@ -62,6 +62,20 @@ trait SelectiveExecutionModule extends mill.define.Module {
       }
     }
 
+  /**
+   * Similar to [[resolve]], but prints the _changed upstream tasks_ rather than
+   * the _selected downstream tasks_.
+   */
+  def resolveChanged(evaluator: Evaluator, tasks: String*): Command[Seq[String]] =
+    Task.Command(exclusive = true) {
+      SelectiveExecution.resolveChanged(evaluator, tasks) match {
+        case Left(err) => Result.Failure(err)
+        case Right(success) =>
+          success.foreach(println)
+          Result.Success(success)
+      }
+    }
+
   /**
    * Run after [[prepare]], selectively executes the tasks in [[tasks]] that are
    * affected by any changes to the task inputs or task implementations since [[prepare]]