add test to show incremental compilation works under pipelining

sbt-test/pipelining/pipelining-changes/build.sbt

@@ -0,0 +1,27 @@
+import sbt.internal.inc.Analysis
+import complete.DefaultParsers._
+
+ThisBuild / usePipelining := true
+
+// Reset compiler iterations, necessary because tests run in batch mode
+val recordPreviousIterations = taskKey[Unit]("Record previous iterations.")
+recordPreviousIterations := {
+  val log = streams.value.log
+  CompileState.previousIterations = {
+    val previousAnalysis = (previousCompile in Compile).value.analysis.asScala
+    previousAnalysis match {
+      case None =>
+        log.info("No previous analysis detected")
+        0
+      case Some(a: Analysis) => a.compilations.allCompilations.size
+    }
+  }
+}
+
+val checkIterations = inputKey[Unit]("Verifies the accumulated number of iterations of incremental compilation.")
+
+checkIterations := {
+  val expected: Int = (Space ~> NatBasic).parsed
+  val actual: Int = ((compile in Compile).value match { case a: Analysis => a.compilations.allCompilations.size }) - CompileState.previousIterations
+  assert(expected == actual, s"Expected $expected compilations, got $actual (previous: ${CompileState.previousIterations})")
+}

sbt-test/pipelining/pipelining-changes/changes/A1.scala

@@ -0,0 +1,5 @@
+package a
+
+enum A {
+  case A, B
+}

sbt-test/pipelining/pipelining-changes/project/CompileState.scala

@@ -0,0 +1,4 @@
+// This is necessary because tests are run in batch mode
+object CompileState {
+  @volatile var previousIterations: Int = -1
+}

sbt-test/pipelining/pipelining-changes/project/DottyInjectedPlugin.scala

@@ -0,0 +1,11 @@
+import sbt._
+import Keys._
+
+object DottyInjectedPlugin extends AutoPlugin {
+  override def requires = plugins.JvmPlugin
+  override def trigger = allRequirements
+
+  override val projectSettings = Seq(
+    scalaVersion := sys.props("plugin.scalaVersion"),
+  )
+}

sbt-test/pipelining/pipelining-changes/src/main/scala/a/A.scala

@@ -0,0 +1,5 @@
+package a
+
+enum A {
+  case A
+}

sbt-test/pipelining/pipelining-changes/src/main/scala/a/App.scala

@@ -0,0 +1,11 @@
+package a
+
+import scala.deriving.Mirror
+
+object App {
+  val m = summon[Mirror.SumOf[a.A]]
+  def size = compiletime.constValue[Tuple.Size[m.MirroredElemTypes]]
+
+  @main def test =
+    assert(size == 2, s"Expected size 2, got $size")
+}

sbt-test/pipelining/pipelining-changes/test

@@ -0,0 +1,7 @@
+# test the interaction of incremental compilation and pipelining
+> compile
+> recordPreviousIterations
+$ copy-file changes/A1.scala src/main/scala/a/A.scala
+# A recompilation should trigger recompilation of App.scala, otherwise test assert will fail
+> run
+> checkIterations 2