Add support for the new Scala 3.8 REPL (#3936)

Author: Gedochao

modules/build/src/main/scala/scala/build/ReplArtifacts.scala

@@ -93,16 +93,28 @@ object ReplArtifacts {
     addScalapy: Option[String],
     javaVersion: Int
   ): Either[BuildException, ReplArtifacts] = either {
-    val isScala2 = scalaParams.scalaVersion.startsWith("2.")
-    val replDep  =
-      if isScala2 then dep"org.scala-lang:scala-compiler:${scalaParams.scalaVersion}"
-      else dep"org.scala-lang::scala3-compiler:${scalaParams.scalaVersion}"
+    val isScala2             = scalaParams.scalaVersion.startsWith("2.")
+    val firstNewReplNightly  = "3.8.0-RC1-bin-20251101-389483e-NIGHTLY".coursierVersion
+    val firstNewReplRc       = "3.8.0-RC1".coursierVersion
+    val firstNewReplStable   = "3.8.0".coursierVersion
+    val scalaCoursierVersion = scalaParams.scalaVersion.coursierVersion
+    val shouldUseNewRepl     =
+      !isScala2 &&
+      ((scalaCoursierVersion >= firstNewReplNightly) || (scalaCoursierVersion >= firstNewReplRc) || scalaCoursierVersion >= firstNewReplStable)
+    val replDeps =
+      if isScala2 then Seq(dep"org.scala-lang:scala-compiler:${scalaParams.scalaVersion}")
+      else if shouldUseNewRepl then
+        Seq(
+          dep"org.scala-lang::scala3-compiler:${scalaParams.scalaVersion}",
+          dep"org.scala-lang::scala3-repl:${scalaParams.scalaVersion}"
+        )
+      else Seq(dep"org.scala-lang::scala3-compiler:${scalaParams.scalaVersion}")
     val scalapyDeps =
       addScalapy.map(ver => dep"${Artifacts.scalaPyOrganization(ver)}::scalapy-core::$ver").toSeq
     val externalDeps                          = dependencies ++ scalapyDeps
     val replArtifacts: Seq[(String, os.Path)] = value {
       Artifacts.artifacts(
-        Seq(replDep).map(Positioned.none),
+        replDeps.map(Positioned.none),
         repositories,
         Some(scalaParams),
         logger,

modules/integration/src/test/scala/scala/cli/integration/ReplTests3NextRc.scala

@@ -1,3 +1,25 @@
 package scala.cli.integration
 
-class ReplTests3NextRc extends ReplTestDefinitions with Test3NextRc
+import com.eed3si9n.expecty.Expecty.expect
+
+class ReplTests3NextRc extends ReplTestDefinitions with Test3NextRc {
+  test("run hello world from the 3.8 REPL") {
+    TestInputs.empty.fromRoot { root =>
+      val expectedMessage = "1337"
+      val code            = s"""println($expectedMessage)"""
+      val r               = os.proc(
+        TestUtil.cli,
+        "repl",
+        "--repl-quit-after-init",
+        "--repl-init-script",
+        code,
+        "-S",
+        // TODO: switch this test to 3.8.0-RC1 once it's out
+        "3.8.0-RC1-bin-20251104-b83b3d9-NIGHTLY",
+        TestUtil.extraOptions
+      )
+        .call(cwd = root)
+      expect(r.out.trim() == expectedMessage)
+    }
+  }
+}