Implement support for Scala 3.8 (#5751)

Instead of using a scala-library-2.13.x, since Scala 3.8.0 the used
scala-library has the same version as the Scala version.

Fix #5739

Co-authored-by: Natsu Kagami <[email protected]>

Author: lefou

libs/javalib/api/src/mill/javalib/api/JvmWorkerUtil.scala

@@ -42,7 +42,7 @@ object JvmWorkerUtil {
       ))
   }
 
-  val PartialVersion: Regex = raw"""(\d+)\.(\d+)\.*""".r
+  val PartialVersion: Regex = raw"""(\d+)\.(\d+)\..*""".r
   val ReleaseVersion: Regex = raw"""(\d+)\.(\d+)\.(\d+)""".r
   val MinorSnapshotVersion: Regex = raw"""(\d+)\.(\d+)\.([1-9]\d*)-SNAPSHOT""".r
   val DottyVersion: Regex = raw"""0\.(\d+)\.(\d+).*""".r
@@ -63,6 +63,10 @@ object JvmWorkerUtil {
     case _ => scalaVersion
   }
 
+  private def minorMajorVersion(version: String): (major: Int, minor: Int) = version match {
+    case PartialVersion(major, minor) => (major = major.toInt, minor = minor.toInt)
+  }
+
   private val ScalaJSFullVersion = """^([0-9]+)\.([0-9]+)\.([0-9]+)(-.*)?$""".r
 
   def scalaJSBinaryVersion(scalaJSVersion: String): String = scalaJSVersion match {
@@ -146,4 +150,22 @@ object JvmWorkerUtil {
       all.filter(v => v.nonEmpty && v >= versionParts.take(v.length)).map(_.mkString(".") + "-")
     (plus ++ minus).distinct.toSeq
   }
+
+  /**
+   * Checks whether the version of the scala-library should be `2.13.x` or just match the given `scalaVersion`
+   *
+   * Background:
+   * To help in Scala 3 adoption and ensure binary compatibility,
+   * Scala 3.x up to 3.7.x used the same Scala standard library as Scala 2.13.
+   * Since this also hinders further improvements of the standard library,
+   * Scala 3.8 will ship with a newer library built with a Scala 3 compiler.
+   *
+   * @param scalaVersion The Scala version
+   * @return `true` if the scala-library version should be enforced to be a `2.13.`
+   */
+  def enforceScala213Library(scalaVersion: String): Boolean = {
+    val sv = minorMajorVersion(scalaVersion)
+    // Some Dotty versions and all Scala 3 versions before 3.8
+    sv.major == 0 || (sv.major == 3 && sv.minor < 8)
+  }
 }

libs/javalib/worker/src/mill/javalib/zinc/ZincWorker.scala

@@ -106,9 +106,10 @@ class ZincWorker(
           loaderLibraryOnly = ClasspathUtil.rootLoader,
           libraryJars = Array(libraryJarNameGrep(
             compilerClasspath,
-            // we don't support too outdated dotty versions
-            // and because there will be no scala 2.14, so hardcode "2.13." here is acceptable
-            if (JvmWorkerUtil.isDottyOrScala3(key.scalaVersion)) "2.13." else key.scalaVersion
+            // if Dotty or Scala 3.0 - 3.7, use the 2.13 version of the standard library
+            if (JvmWorkerUtil.enforceScala213Library(key.scalaVersion)) "2.13."
+            // otherwise use the library matching the Scala version
+            else key.scalaVersion
           ).path.toIO),
           compilerJars = combinedCompilerJars,
           allJars = combinedCompilerJars,

libs/scalalib/test/src/mill/scalalib/Scala38NightlyTests.scala

@@ -0,0 +1,40 @@
+package mill.scalalib
+
+import mill.*
+import mill.api.{Discover, ModuleRef}
+import mill.testkit.{TestRootModule, UnitTester}
+import utest.*
+
+// TODO: Once Scala 3.8.0 is out, we can change this test to use 3.8.0 and remove the extra repo
+object Scala38NightlyTests extends TestSuite {
+
+  val repo = "https://repo.scala-lang.org/artifactory/maven-nightlies"
+
+  object Scala38Nightly extends TestRootModule {
+    object JvmWorker extends JvmWorkerModule {
+      override def repositories = super.repositories() ++ Seq(repo)
+    }
+    object foo extends ScalaModule {
+      override def jvmWorker: ModuleRef[JvmWorkerModule] = ModuleRef(JvmWorker)
+      override def repositories = super.repositories() ++ Seq(repo)
+      override def scalaVersion = "3.8.0-RC1-bin-20250825-ee2f641-NIGHTLY"
+      override def mvnDeps = Seq(
+        mvn"org.scala-lang.modules::scala-xml:2.4.0"
+      )
+    }
+
+    lazy val millDiscover = Discover[this.type]
+  }
+
+  def tests: Tests = Tests {
+
+    test("scala38nightly") - UnitTester(
+      Scala38Nightly,
+      sourceRoot = os.Path(sys.env("MILL_TEST_RESOURCE_DIR")) / "dotty213"
+    ).scoped { eval =>
+      val Right(result) = eval.apply(Scala38Nightly.foo.run()): @unchecked
+      assert(result.evalCount > 0)
+    }
+
+  }
+}

libs/scalalib/test/src/mill/scalalib/ScalaDotty213Tests.scala

@@ -9,8 +9,9 @@ object ScalaDotty213Tests extends TestSuite {
   object Dotty213 extends TestRootModule {
     object foo extends ScalaModule {
       def scalaVersion = "0.18.1-RC1"
-      override def mvnDeps =
-        Seq(mvn"org.scala-lang.modules::scala-xml:1.2.0".withDottyCompat(scalaVersion()))
+      override def mvnDeps = Seq(
+        mvn"org.scala-lang.modules::scala-xml:1.2.0".withDottyCompat(scalaVersion())
+      )
     }
 
     lazy val millDiscover = Discover[this.type]