bugfix: Keep coverage data in the class directory on sbt 2

.github/workflows/ci.yml

@@ -38,6 +38,12 @@ jobs:
         if: "${{ matrix.java != 8 }}"
         run: sbt "++ 3.x" test
 
+      # "++ 3.x" selects the sbt 2 plugin build (pluginCrossBuild / sbtVersion), so scripted
+      # launches the test projects with sbt 2. Only tests that support sbt 2 are listed.
+      - name: Run scripted tests sbt 2
+        if: "${{ matrix.java != 8 }}"
+        run: sbt "++ 3.x" "scripted scoverage/cached-compile"
+
   formatting:
     runs-on: ubuntu-latest
 

README.md

@@ -135,7 +135,11 @@ settings will apply to that report.
 ### Override Location for Coverage Data And Report
 
 If desired, one could override the default location for generating the
-sbt report and data through setting `coverageDataDir`:
+sbt report and data through setting `coverageDataDir`. It defaults to
+`crossTarget` on sbt 1 and to `Compile / classDirectory` on sbt 2. On sbt 2,
+`compile` is cached, and a cache hit only restores the class directory, so the
+scoverage data must live inside it. If you override it on sbt 2, keep it inside
+`Compile / classDirectory`.
 
 Example in data-dir test:
 ```scala

src/main/scala-2/scoverage/ScoverageSbtPluginCompat.scala

@@ -1,7 +1,14 @@
 package scoverage
 
+import sbt.Keys._
+import sbt._
+
 private[scoverage] object ScoverageSbtPluginCompat {
   implicit class DefOps(private val self: sbt.Def.type) extends AnyVal {
     def uncached[A](a: A): A = a
   }
+
+  /** Default for `coverageDataDir` on sbt 1. */
+  val defaultCoverageDataDir: Def.Initialize[File] =
+    Def.setting(crossTarget.value)
 }

src/main/scala-3/scoverage/ScoverageSbtPluginCompat.scala

@@ -1,3 +1,22 @@
 package scoverage
 
-private[scoverage] object ScoverageSbtPluginCompat
+import sbt.Keys.*
+import sbt.*
+
+private[scoverage] object ScoverageSbtPluginCompat {
+
+  /** Default for `coverageDataDir` on sbt 2.
+    *
+    * sbt 2 caches `compile` and, on a cache hit, restores only its declared
+    * outputs, i.e. the class directory. The scoverage-data directory
+    * (`scoverage.coverage`, plus the directory the instrumented classes write
+    * measurements into) is a side effect of compilation. Under `crossTarget` it
+    * is not restored, so tests fail with a FileNotFoundException or
+    * coverageReport finds no data. Keeping it inside the class directory makes
+    * it part of the cached compile output.
+    *
+    * See https://github.com/scoverage/sbt-scoverage/issues/696
+    */
+  val defaultCoverageDataDir: Def.Initialize[File] =
+    Def.setting((Compile / classDirectory).value)
+}

src/main/scala/scoverage/ScoverageSbtPlugin.scala

@@ -71,7 +71,7 @@ object ScoverageSbtPlugin extends AutoPlugin {
     coverageReport := coverageReport0.value,
     coverageAggregate := coverageAggregate0.value,
     coverageAggregate / aggregate := false,
-    coverageDataDir := crossTarget.value,
+    coverageDataDir := defaultCoverageDataDir.value,
     coverageScalacPluginVersion := {
       scalaVersion.value match {
         case "2.13.11" => "2.3.0"

src/sbt-test/scoverage/cached-compile/build.sbt

@@ -0,0 +1,16 @@
+version := "0.1"
+scalaVersion := "3.8.4"
+libraryDependencies += "org.scalameta" %% "munit" % "1.1.1" % Test
+Test / fork := true
+coverageMinimumStmtTotal := 100
+coverageFailOnMinimum := true
+
+// coverageReport only logs "No coverage data" when the instrumentation file is
+// missing, so assert on it explicitly. A command rather than a task: sbt 2
+// caches tasks, and a cached check would not look at the file again.
+commands += Command.command("checkCoverageData") { state =>
+  val dir = Project.extract(state).get(coverageDataDir)
+  val f = dir / "scoverage-data" / "scoverage.coverage"
+  if (!f.exists) sys.error(s"missing $f")
+  state
+}

src/sbt-test/scoverage/cached-compile/project/plugins.sbt

@@ -0,0 +1,9 @@
+val pluginVersion = sys.props.getOrElse(
+  "plugin.version",
+  throw new RuntimeException(
+    """|The system property 'plugin.version' is not defined.
+       |Specify this property using the scriptedLaunchOpts -D.""".stripMargin
+  )
+)
+
+addSbtPlugin("org.scoverage" % "sbt-scoverage" % pluginVersion)

src/sbt-test/scoverage/cached-compile/src/main/scala/Calc.scala

@@ -0,0 +1,3 @@
+object Calc {
+  def add(a: Int, b: Int): Int = a + b
+}

src/sbt-test/scoverage/cached-compile/src/test/scala/CalcSuite.scala

@@ -0,0 +1,3 @@
+class CalcSuite extends munit.FunSuite {
+  test("add")(assertEquals(Calc.add(1, 2), 3))
+}

src/sbt-test/scoverage/cached-compile/test

@@ -0,0 +1,14 @@
+# First round compiles with coverage and populates the build cache.
+> clean
+> coverage
+> testOnly CalcSuite
+> checkCoverageData
+> coverageReport
+
+# Second round: on sbt 2, compile is restored from the cache after clean.
+# The scoverage data must come back with it.
+> clean
+> coverage
+> testOnly CalcSuite
+> checkCoverageData
+> coverageReport