improvement: if no build tool fallback to scala-cli

metals/src/main/scala/scala/meta/internal/metals/MetalsEnrichments.scala

@@ -1,5 +1,6 @@
 package scala.meta.internal.metals
 
+import java.io.File
 import java.io.IOException
 import java.net.URI
 import java.nio.charset.StandardCharsets
@@ -294,6 +295,16 @@ object MetalsEnrichments
 
   implicit class XtensionAbsolutePathBuffers(path: AbsolutePath) {
 
+    def hasScalaFiles: Boolean = {
+      def isScalaDir(file: File): Boolean = {
+        file.listFiles().exists { file =>
+          if (file.isDirectory()) isScalaDir(file)
+          else file.getName().endsWith(".scala")
+        }
+      }
+      path.isDirectory && isScalaDir(path.toFile)
+    }
+
     def scalaSourcerootOption: String = s""""-P:semanticdb:sourceroot:$path""""
 
     def javaSourcerootOption: String =

metals/src/main/scala/scala/meta/internal/metals/MetalsLspService.scala

@@ -882,17 +882,20 @@ class MetalsLspService(
   def connectTables(): Connection = tables.connect()
 
   def initialized(): Future[Unit] =
-    Future
-      .sequence(
-        List[Future[Unit]](
-          Future(buildTools.initialize()),
-          quickConnectToBuildServer().ignoreValue,
-          slowConnectToBuildServer(forceImport = false).ignoreValue,
-          Future(workspaceSymbols.indexClasspath()),
-          Future(formattingProvider.load()),
-        )
-      )
-      .ignoreValue
+    for {
+      _ <- maybeSetupScalaCli()
+      _ <-
+        Future
+          .sequence(
+            List[Future[Unit]](
+              Future(buildTools.initialize()),
+              quickConnectToBuildServer().ignoreValue,
+              slowConnectToBuildServer(forceImport = false).ignoreValue,
+              Future(workspaceSymbols.indexClasspath()),
+              Future(formattingProvider.load()),
+            )
+          )
+    } yield ()
 
   def onShutdown(): Unit = {
     tables.fingerprints.save(fingerprints.getAllFingerprints())
@@ -1941,7 +1944,16 @@ class MetalsLspService(
               tables.buildServers.chooseServer(ScalaCliBuildTool.name)
               quickConnectToBuildServer()
             case Some(digest) if isBloopOrEmpty =>
-              slowConnectToBloopServer(forceImport, buildTool, digest)
+              for {
+                _ <-
+                  if (scalaCli.loaded(folder)) scalaCli.stop()
+                  else Future.successful(())
+                buildChange <- slowConnectToBloopServer(
+                  forceImport,
+                  buildTool,
+                  digest,
+                )
+              } yield buildChange
             case Some(digest) =>
               indexer.reloadWorkspaceAndIndex(
                 forceImport,
@@ -1955,6 +1967,20 @@ class MetalsLspService(
       }
     } yield buildChange
 
+  /**
+   * If there is no auto-connectable build server and no supported build tool is found
+   * we assume it's a scala-cli project.
+   */
+  def maybeSetupScalaCli(): Future[Unit] = {
+    if (
+      !buildTools.isAutoConnectable
+      && buildTools.loadSupported.isEmpty
+      && folder.hasScalaFiles
+    ) {
+      scalaCli.setupIDE(folder)
+    } else Future.successful(())
+  }
+
   private def slowConnectToBloopServer(
       forceImport: Boolean,
       buildTool: BuildTool,

metals/src/main/scala/scala/meta/internal/metals/scalacli/ScalaCli.scala

@@ -152,7 +152,7 @@ class ScalaCli(
     }
   }
 
-  private lazy val baseCommand = {
+  private lazy val localScalaCli: Option[Seq[String]] = {
 
     def endsWithCaseInsensitive(s: String, suffix: String): Boolean =
       s.length >= suffix.length &&
@@ -217,7 +217,7 @@ class ScalaCli(
       version.exists(ver => minVersion0.compareTo(ver) <= 0)
     }
 
-    val cliCommand = userConfig().scalaCliLauncher
+    userConfig().scalaCliLauncher
       .filter(_.trim.nonEmpty)
       .map(Seq(_))
       .orElse {
@@ -226,34 +226,65 @@ class ScalaCli(
           .filter(requireMinVersion(_, ScalaCli.minVersion))
           .map(p => Seq(p.toString))
       }
-      .getOrElse {
-        scribe.warn(
-          s"scala-cli >= ${ScalaCli.minVersion} not found in PATH, fetching and starting a JVM-based Scala CLI"
-        )
-        val cp = ScalaCli.scalaCliClassPath()
-        Seq(
-          ScalaCli.javaCommand,
-          "-cp",
-          cp.mkString(File.pathSeparator),
-          ScalaCli.scalaCliMainClass,
-        )
-      }
-    cliCommand ++ Seq("bsp")
+  }
+
+  private lazy val cliCommand = {
+    localScalaCli.getOrElse {
+      scribe.warn(
+        s"scala-cli >= ${ScalaCli.minVersion} not found in PATH, fetching and starting a JVM-based Scala CLI"
+      )
+      jvmBased()
+    }
+  }
+
+  def jvmBased(): Seq[String] = {
+    val cp = ScalaCli.scalaCliClassPath()
+    Seq(
+      ScalaCli.javaCommand,
+      "-cp",
+      cp.mkString(File.pathSeparator),
+      ScalaCli.scalaCliMainClass,
+    )
   }
 
   def loaded(path: AbsolutePath): Boolean =
     ifConnectedOrElse(st =>
       st.path == path || path.toNIO.startsWith(st.path.toNIO)
     )(false)
 
+  def setupIDE(path: AbsolutePath): Future[Unit] = {
+    localScalaCli
+      .map { cliCommand =>
+        val command = cliCommand ++ Seq("setup-ide", path.toString())
+        scribe.info(s"Running $command")
+        val proc = SystemProcess.run(
+          command.toList,
+          path,
+          redirectErrorOutput = false,
+          env = Map(),
+          processOut = None,
+          processErr = Some(line => scribe.info("Scala CLI: " + line)),
+          discardInput = false,
+          threadNamePrefix = "scala-cli-setup-ide",
+        )
+        proc.complete.ignoreValue
+      }
+      .getOrElse {
+        start(path)
+      }
+  }
+
+  def path: Option[AbsolutePath] =
+    ifConnectedOrElse(st => Option(st.path))(None)
+
   def start(path: AbsolutePath): Future[Unit] = {
     disconnectOldBuildServer().onComplete {
       case Failure(e) =>
         scribe.warn("Error disconnecting old Scala CLI server", e)
       case Success(()) =>
     }
 
-    val command = baseCommand :+ path.toString()
+    val command = cliCommand :+ "bsp" :+ path.toString()
 
     val connDir = if (path.isDirectory) path else path.parent
 
@@ -283,7 +314,7 @@ class ScalaCli(
           scribe.error("Error starting Scala CLI", ex)
           Success(())
         case Success(_) =>
-          scribe.info("Scala CLI started")
+          scribe.info(s"Scala CLI started for $path")
           Success(())
       }
     } else {

tests/slow/src/test/scala/tests/feature/SyntaxErrorLspSuite.scala

@@ -161,7 +161,7 @@ class SyntaxErrorLspSuite extends BaseLspSuite("syntax-error") {
     } yield ()
   }
 
-  test("no-build-tool") {
+  test("no-build-tool") { // we fallback to scala-cli
     for {
       _ <- initialize(
         """
@@ -173,7 +173,10 @@ class SyntaxErrorLspSuite extends BaseLspSuite("syntax-error") {
       _ <- server.didOpen("A.scala")
       _ = assertNoDiff(
         client.workspaceDiagnostics,
-        """|A.scala:1:20: error: illegal start of simple expression
+        """|A.scala:1:20: error: expression expected but '}' found
+           |object A { val x = }
+           |                   ^
+           |A.scala:1:20: error: illegal start of simple expression
            |object A { val x = }
            |                   ^
            |""".stripMargin,

tests/slow/src/test/scala/tests/scalacli/ScalaCliSuite.scala

@@ -184,6 +184,23 @@ class ScalaCliSuite extends BaseScalaCliSuite(V.scala3) {
     } yield ()
   }
 
+
+  test("connecting-scalacli-as-fallback") {
+     cleanWorkspace()
+     FileLayout.fromString(simpleFileLayout, workspace)
+     for {
+       _ <- server.initialize()
+       _ <- server.initialized()
+       _ <- server.server.indexingPromise.future
+       _ <- server.didOpen("MyTests.scala")
+       _ <- assertDefinitionAtLocation(
+         "MyTests.scala",
+         "new Fo@@o",
+         "foo.sc",
+       )
+     } yield ()
+  }
+
   test("relative-semanticdb-root") {
     for {
       _ <- scalaCliInitialize(useBsp = false)(