bugfix: Use proper rt.jar if it exists

backend/src/main/scala/bloop/Compiler.scala

@@ -873,9 +873,19 @@ object Compiler {
       case Array("-release", "8") => true
       case _ => false
     }
-    val updatedClasspath =
-      if (needsRtJar) inputs.classpath ++ RtJarCache.create(JavaRuntime.version, logger)
-      else inputs.classpath
+    val possibleRtJar =
+      if (needsRtJar)
+        inputs.javacBin
+          .flatMap { binary =>
+            Try {
+              val javaHome = binary.getParent.getParent
+              javaHome.resolve("jre/lib/rt.jar")
+            }.toOption
+          }
+          .filter(_.exists)
+          .orElse(RtJarCache.create(JavaRuntime.version, logger))
+      else None
+    val updatedClasspath = inputs.classpath ++ possibleRtJar
     val classpathVirtual = updatedClasspath.map(path => converter.toVirtualFile(path.underlying))
     CompileOptions
       .create()

frontend/src/test/scala/bloop/JavaVersionSpec.scala

@@ -11,7 +11,7 @@ object JavaVersionSpec extends bloop.testing.BaseSuite {
 
   private val jvmManager = coursierapi.JvmManager.create()
 
-  def checkFlag(scalacOpts: List[String], jdkVersion: String = "8") = {
+  def checkFlag(scalacOpts: List[String], jdkVersion: String = "8", shouldFail: Boolean = false) = {
     val javaHome = jvmManager.get(jdkVersion).toPath()
     val jvmConfig = Some(Config.JvmConfig(Some(javaHome), Nil))
     TestUtil.withinWorkspace { workspace =>
@@ -29,7 +29,7 @@ object JavaVersionSpec extends bloop.testing.BaseSuite {
       val projects = List(`A`)
       val state = loadState(workspace, projects, logger)
       val compiledState = state.compile(`A`)
-      if (jdkVersion == "8") {
+      if (jdkVersion == "8" || shouldFail) {
         assertExitStatus(compiledState, ExitStatus.CompilationError)
         val targetFoo = TestUtil.universalPath("a/src/main/scala/Foo.scala")
         assertNoDiff(
@@ -63,7 +63,84 @@ object JavaVersionSpec extends bloop.testing.BaseSuite {
     checkFlag(Nil, jdkVersion = "11")
   }
 
+  test("doesnt-compile-with-11") {
+    checkFlag(List("-release", "8"), jdkVersion = "11", shouldFail = true)
+    checkFlag(List("-release:8"), jdkVersion = "11", shouldFail = true)
+  }
+
   test("compiles-with-17") {
     checkFlag(Nil, jdkVersion = "17")
   }
+
+  test("doesnt-compile-with-17") {
+    checkFlag(List("-release", "8"), jdkVersion = "17", shouldFail = true)
+    checkFlag(List("-release:8"), jdkVersion = "17", shouldFail = true)
+  }
+
+  def checkRtJar(jdkVersion: String, scalacOpts: List[String] = Nil) = {
+    val javaHome = jvmManager.get(jdkVersion).toPath()
+    val jvmConfig = Some(Config.JvmConfig(Some(javaHome), Nil))
+    TestUtil.withinWorkspace { workspace =>
+      val sources = List(
+        """/main/scala/Foo.scala
+          |import java.net.http.HttpClient
+          |class Foo{
+          |  val output: HttpClient = ???
+          |}
+          """.stripMargin
+      )
+
+      val logger = new RecordingLogger(ansiCodesSupported = false)
+      val `A` =
+        TestProject(workspace, "a", sources, jvmConfig = jvmConfig, scalacOptions = scalacOpts)
+      val projects = List(`A`)
+      val state = loadState(workspace, projects, logger)
+      val compiledState = state.compile(`A`)
+      if (jdkVersion == "8") {
+        assertExitStatus(compiledState, ExitStatus.CompilationError)
+        val targetFoo = TestUtil.universalPath("a/src/main/scala/Foo.scala")
+        assertNoDiff(
+          logger.renderErrors(),
+          s"""|[E2] $targetFoo:3:15
+              |     not found: type HttpClient
+              |     L3:   val output: HttpClient = ???
+              |                       ^
+              |[E1] $targetFoo:1:17
+              |     object http is not a member of package java.net
+              |     L1: import java.net.http.HttpClient
+              |                         ^
+              |$targetFoo: L1 [E1], L3 [E2]
+              |Failed to compile 'a'
+              |""".stripMargin
+        )
+      } else {
+        assertExitStatus(compiledState, ExitStatus.Ok)
+      }
+    }
+  }
+
+  if (!CrossPlatform.isM1) {
+    test("doesnt-compile") {
+      checkRtJar(jdkVersion = "8")
+    }
+  }
+
+  test("compiles-11") {
+    checkRtJar(jdkVersion = "11")
+
+  }
+
+  test("compiles-17") {
+    checkRtJar(jdkVersion = "17")
+  }
+
+  /* Current limitation: this should fail but the generated rt.jar contains all JDK 11 classes.
+   * However, most tools generating json configuration will set proper javaHome with JDK 8.
+   * Only if a user sets the flags themselves this will wrongly compile classes coming
+   * from future JDKs (but not added APIs).
+   */
+  test("compiles-wrongly") {
+    checkRtJar(jdkVersion = "11", List("-release", "8"))
+    checkRtJar(jdkVersion = "17", List("-release", "8"))
+  }
 }