Refactor to definedTestDigests task

main-actions/src/main/scala/sbt/Tests.scala

@@ -307,6 +307,7 @@ object Tests {
     in.filter(t => seen.add(f(t)))
   }
 
+  // Called by Defaults
   def apply(
       frameworks: Map[TestFramework, Framework],
       testLoader: ClassLoader,
@@ -340,7 +341,7 @@ object Tests {
     apply(frameworks, testLoader, runners, o, config, log)
   }
 
-  def testTask(
+  private[sbt] def testTask(
       loader: ClassLoader,
       frameworks: Map[TestFramework, Framework],
       runners: Map[TestFramework, Runner],

main/src/main/scala/sbt/Defaults.scala

@@ -1329,6 +1329,9 @@ object Defaults extends BuildCommon {
         .storeAs(definedTestNames)
         .triggeredBy(compile)
         .value,
+      definedTestDigests := IncrementalTest.definedTestDigestTask
+        .triggeredBy(compile)
+        .value,
       testQuick / testFilter := IncrementalTest.filterTask.value,
       executeTests := {
         import sbt.TupleSyntax.*
@@ -1411,8 +1414,7 @@ object Defaults extends BuildCommon {
           ) +:
             TestStatusReporter(
               IncrementalTest.succeededFile((test / streams).value.cacheDirectory),
-              (Keys.test / fullClasspath).value,
-              fileConverter.value,
+              definedTestDigests.value,
             ) +:
             (TaskZero / testListeners).value
         },

main/src/main/scala/sbt/Keys.scala

@@ -37,7 +37,7 @@ import sbt.librarymanagement._
 import sbt.librarymanagement.ivy.{ Credentials, IvyConfiguration, IvyPaths, UpdateOptions }
 import sbt.nio.file.Glob
 import sbt.testing.Framework
-import sbt.util.{ cacheLevel, ActionCacheStore, Level, Logger, LoggerContext }
+import sbt.util.{ cacheLevel, ActionCacheStore, Digest, Level, Logger, LoggerContext }
 import xsbti.{ HashedVirtualFileRef, VirtualFile, VirtualFileRef }
 import xsbti.compile._
 import xsbti.compile.analysis.ReadStamps
@@ -349,8 +349,10 @@ object Keys {
   // Test Keys
   val testLoader = taskKey[ClassLoader]("Provides the class loader used for testing.").withRank(DTask)
   val loadedTestFrameworks = taskKey[Map[TestFramework, Framework]]("Loads Framework definitions from the test loader.").withRank(DTask)
+  @cacheLevel(include = Array.empty)
   val definedTests = taskKey[Seq[TestDefinition]]("Provides the list of defined tests.").withRank(BMinusTask)
   val definedTestNames = taskKey[Seq[String]]("Provides the set of defined test names.").withRank(BMinusTask)
+  val definedTestDigests = taskKey[Map[String, Digest]]("Provides a unique digest of defined tests.").withRank(DTask)
   val executeTests = taskKey[Tests.Output]("Executes all tests, producing a report.").withRank(CTask)
   val test = taskKey[Unit]("Executes all tests.").withRank(APlusTask)
   val testOnly = inputKey[Unit]("Executes the tests provided as arguments or all tests if no arguments are provided.").withRank(ATask)

main/src/main/scala/sbt/internal/IncrementalTest.scala

@@ -11,7 +11,7 @@ package internal
 
 import java.io.File
 import java.util.concurrent.ConcurrentHashMap
-import Keys.{ test, compileInputs, fileConverter, fullClasspath, streams }
+import Keys.{ test, fileConverter, fullClasspath, streams }
 import sbt.Def.Initialize
 import sbt.internal.inc.Analysis
 import sbt.internal.util.Attributed
@@ -32,17 +32,30 @@ object IncrementalTest:
     Def.task {
       val cp = (Keys.test / fullClasspath).value
       val s = (Keys.test / streams).value
-      val converter = fileConverter.value
-      val stamper = ClassStamper(cp, converter)
+      val digests = (Keys.definedTestDigests).value
       val succeeded = TestStatus.read(succeededFile(s.cacheDirectory))
       def hasSucceeded(className: String): Boolean = succeeded.get(className) match
         case None     => false
-        case Some(ts) => ts == stamper.transitiveStamp(className)
+        case Some(ts) => Some(ts) == digests.get(className)
       args =>
         for filter <- selectedFilter(args)
         yield (test: String) => filter(test) && !hasSucceeded(test)
     }
 
+  // cache the test digests against the fullClasspath.
+  def definedTestDigestTask: Initialize[Task[Map[String, Digest]]] = Def.cachedTask {
+    val cp = (Keys.test / fullClasspath).value
+    val testNames = Keys.definedTests.value.map(_.name).toVector.distinct
+    val converter = fileConverter.value
+    val stamper = ClassStamper(cp, converter)
+    // TODO: Potentially do something about JUnit 5 and others which might not use class name
+    Map((testNames.flatMap: name =>
+      stamper.transitiveStamp(name) match
+        case Some(ts) => Seq(name -> ts)
+        case None     => Nil
+    ): _*)
+  }
+
   def succeededFile(dir: File): File = dir / "succeeded_tests.txt"
 
   def selectedFilter(args: Seq[String]): Seq[String => Boolean] =
@@ -123,9 +136,10 @@ class ClassStamper(
   /**
    * Given a classpath and a class name, this tries to create a SHA-256 digest.
    */
-  def transitiveStamp(className: String): Digest =
+  def transitiveStamp(className: String): Option[Digest] =
     val digests = SortedSet(analyses.flatMap(internalStamp(className, _, Set.empty)): _*)
-    Digest.sha256Hash(digests.toSeq: _*)
+    if digests.nonEmpty then Some(Digest.sha256Hash(digests.toSeq: _*))
+    else None
 
   private def internalStamp(
       className: String,
@@ -144,7 +158,7 @@ class ClassStamper(
               internalStamp(otherCN, analysis, alreadySeen + className)
           val internalJarDeps = relations
             .externalDeps(className)
-            .map: libClassName =>
+            .flatMap: libClassName =>
               transitiveStamp(libClassName)
           val externalDeps = relations
             .externalDeps(className)

testing/src/main/scala/sbt/TestFramework.scala

@@ -93,6 +93,7 @@ final class TestFramework(val implClassNames: String*) extends Serializable {
   def create(loader: ClassLoader, log: ManagedLogger): Option[Framework] =
     createFramework(loader, log, implClassNames.toList)
 }
+
 final class TestDefinition(
     val name: String,
     val fingerprint: Fingerprint,
@@ -108,7 +109,7 @@ final class TestDefinition(
   override def hashCode: Int = (name.hashCode, TestFramework.hashCode(fingerprint)).hashCode
 }
 
-final class TestRunner(
+private[sbt] final class TestRunner(
     delegate: Runner,
     listeners: Vector[TestReportListener],
     log: ManagedLogger
@@ -214,7 +215,7 @@ object TestFramework {
       case _                        => f.toString
     }
 
-  def testTasks(
+  private[sbt] def testTasks(
       frameworks: Map[TestFramework, Framework],
       runners: Map[TestFramework, Runner],
       testLoader: ClassLoader,