Fix issue #2958 - deduplicate (anonymous) tasks in result (#2959)

Anonymous tasks can be defined as `val` or `def`. This is in contrast to
named targets, which always need to be defined as `def`.

Also, anonymous tasks end up in the terminal group of the target that's
using them.

If the same anonymous task is used in multiple targets, it also ends up
multiple time in the task results. When a `Evaluator.Results` is
created, this clashes, due to the use of a `Strict.Agg` for
`Evalutor.Results.evaluated`.

This change deduplicates the results before constructing the
`Evaluator.Results`.

Pull request: #2959

main/eval/src/mill/eval/EvaluatorCore.scala

@@ -169,7 +169,12 @@ private[mill] trait EvaluatorCore extends GroupEvaluator {
 
     EvaluatorCore.Results(
       goals.indexed.map(results(_).map(_._1).result),
-      finishedOptsMap.map(_._2).flatMap(_.toSeq.flatMap(_.newEvaluated)),
+      // result of flatMap may contain non-distinct entries,
+      // so we manually clean it up before converting to a `Strict.Agg`
+      // see https://github.com/com-lihaoyi/mill/issues/2958
+      Strict.Agg.from(
+        finishedOptsMap.values.flatMap(_.toSeq.flatMap(_.newEvaluated)).iterator.distinct
+      ),
       transitive,
       getFailing(sortedGroups, results),
       results.map { case (k, v) => (k, v.map(_._1)) }

main/eval/test/src/mill/eval/TaskTests.scala

@@ -2,7 +2,7 @@ package mill.eval
 
 import utest._
 import mill.T
-import mill.define.Worker
+import mill.define.{Module, Worker}
 import mill.util.{TestEvaluator, TestUtil}
 import utest.framework.TestPath
 
@@ -118,6 +118,18 @@ trait TaskTests extends TestSuite {
       superBuildTargetOverrideWithInputCount += 1
       superBuildTargetOverrideWithInputCount
     }
+
+    // Reproduction of issue https://github.com/com-lihaoyi/mill/issues/2958
+    object repro2958 extends Module {
+      val task1 = T.task { "task1" }
+      def task2 = T { task1() }
+      def task3 = T { task1() }
+      def command() = T.command {
+        val t2 = task2()
+        val t3 = task3()
+        s"${t2},${t3}"
+      }
+    }
   }
 
   def withEnv(f: (Build, TestEvaluator) => Unit)(implicit tp: TestPath): Unit
@@ -241,7 +253,11 @@ trait TaskTests extends TestSuite {
         check.apply(build.superBuildTargetOverrideWithInput) ==> Right((3, 0))
       }
     }
+    "duplicateTaskInResult-issue2958" - withEnv { (build, check) =>
+      check.apply(build.repro2958.command()) ==> Right(("task1,task1", 3))
+    }
   }
+
 }
 
 object SeqTaskTests extends TaskTests {