Pass default dialect to scalafmt

modules/cli/src/main/scala/scala/cli/commands/Fmt.scala

@@ -2,8 +2,8 @@ package scala.cli.commands
 
 import caseapp._
 
-import scala.build.Inputs
-import scala.build.internal.Runner
+import scala.build.internal.{CustomCodeWrapper, Runner}
+import scala.build.{CrossSources, Inputs, Sources}
 import scala.cli.internal.FetchExternalBinary
 
 object Fmt extends ScalaCommand[FmtOptions] {
@@ -12,16 +12,16 @@ object Fmt extends ScalaCommand[FmtOptions] {
   def run(options: FmtOptions, args: RemainingArgs): Unit = {
 
     // TODO If no input is given, just pass '.' to scalafmt?
-    val (sourceFiles, workspace) =
+    val (sourceFiles, workspace, inputsOpt) =
       if (args.remaining.isEmpty)
-        (Seq(os.pwd), os.pwd)
+        (Seq(os.pwd), os.pwd, None)
       else {
         val i = options.shared.inputsOrExit(args)
         val s = i.sourceFiles().collect {
           case sc: Inputs.Script    => sc.path
           case sc: Inputs.ScalaFile => sc.path
         }
-        (s, i.workspace)
+        (s, i.workspace, Some(i))
       }
 
     val logger = options.shared.logger
@@ -31,6 +31,36 @@ object Fmt extends ScalaCommand[FmtOptions] {
       logger.debug("No source files, not formatting anything")
     else {
 
+      def scalaVerOpt = inputsOpt.map { inputs =>
+        val crossSources =
+          CrossSources.forInputs(
+            inputs,
+            Sources.defaultPreprocessors(
+              options.buildOptions.scriptOptions.codeWrapper.getOrElse(CustomCodeWrapper)
+            )
+          ).orExit(logger)
+        val sharedOptions = crossSources.sharedOptions(options.buildOptions)
+        sharedOptions
+          .scalaParams
+          .orExit(logger)
+          .scalaVersion
+      }
+
+      def dialectOpt = options.dialect.map(_.trim).filter(_.nonEmpty).orElse {
+        scalaVerOpt.flatMap {
+          case v if v.startsWith("2.12.") => Some("Scala212")
+          case v if v.startsWith("2.13.") => Some("Scala213")
+          case v if v.startsWith("3.")    => Some("Scala3")
+          case _                          => None
+        }
+      }
+
+      val dialectArgs =
+        if (options.scalafmtArg.isEmpty && !os.exists(workspace / ".scalafmt.conf"))
+          dialectOpt.toSeq.flatMap(dialect => Seq("--config-str", s"runner.dialect=$dialect"))
+        else
+          Nil
+
       val fmtLauncher = options.scalafmtLauncher.filter(_.nonEmpty) match {
         case Some(launcher) =>
           os.Path(launcher, os.pwd)
@@ -41,7 +71,10 @@ object Fmt extends ScalaCommand[FmtOptions] {
 
       logger.debug(s"Using scalafmt launcher $fmtLauncher")
 
-      val command = Seq(fmtLauncher.toString) ++ sourceFiles.map(_.toString)
+      val command = Seq(fmtLauncher.toString) ++
+        sourceFiles.map(_.toString) ++
+        dialectArgs ++
+        options.scalafmtArg
       Runner.run(
         "scalafmt",
         command,

modules/cli/src/main/scala/scala/cli/commands/FmtOptions.scala

@@ -3,6 +3,7 @@ package scala.cli.commands
 import caseapp._
 
 import scala.build.internal.Constants
+import scala.build.options.BuildOptions
 import scala.cli.internal.FetchExternalBinary
 import scala.util.Properties
 
@@ -11,19 +12,34 @@ import scala.util.Properties
 final case class FmtOptions(
   @Recurse
     shared: SharedOptions = SharedOptions(),
+
+  @Group("Format")
   @HelpMessage("Check that sources are well formatted")
     check: Boolean = false,
 
+  @Group("Format")
   @Hidden
     osArchSuffix: Option[String] = None,
+  @Group("Format")
   @Hidden
     scalafmtTag: Option[String] = None,
+  @Group("Format")
   @Hidden
     scalafmtGithubOrgName: Option[String] = None,
+  @Group("Format")
   @Hidden
     scalafmtExtension: Option[String] = None,
+  @Group("Format")
+  @Hidden
+    scalafmtLauncher: Option[String] = None,
+
+  @Group("Format")
+  @Name("F")
   @Hidden
-    scalafmtLauncher: Option[String] = None
+    scalafmtArg: List[String] = Nil,
+
+  @Group("Format")
+    dialect: Option[String] = None
 ) {
   // format: on
 
@@ -38,6 +54,9 @@ final case class FmtOptions(
     (url, !tag0.startsWith("v"))
   }
 
+  def buildOptions: BuildOptions =
+    shared.buildOptions(enableJmh = false, jmhVersion = None, ignoreErrors = false)
+
 }
 
 object FmtOptions {

website/docs/reference/cli-options.md

@@ -280,6 +280,12 @@ Check that sources are well formatted
 
 #### `--scalafmt-launcher`
 
+#### `--scalafmt-arg`
+
+Aliases: `-F`
+
+#### `--dialect`
+
 ## Help options
 
 Available in commands: