Add WASM support: --wasm flag with Node.js and Deno runtimes

Implements scala-cli issue #3316: integrate WebAssembly with Scala CLI.

- `--wasm` CLI flag and `//> using wasm` directive to enable WASM output
- `--wasm-runtime <runtime>` option and `//> using wasmRuntime` directive
  Supported values: node (default), deno
- `--deno-version`, `--wasmtime-version`, `--wasmer-version` options
  and corresponding directives for pinning runtime versions

- **Node.js** (default): runs Scala.js WASM output with
  `--experimental-wasm-exnref` flag, requires Node.js >= 22
- **Deno**: runs Scala.js WASM output

Author: lostflydev

build.mill

@@ -533,6 +533,8 @@ trait Core extends ScalaCliCrossSbtModule
          |  def toolkitVersionForNative04 = "${Deps.toolkitVersionForNative04}"
          |  def toolkitVersionForNative05 = "${Deps.toolkitVersionForNative05}"
          |
+         |  def defaultDenoVersion = "2.1.4"
+         |
          |  def typelevelOrganization = "${Deps.typelevelToolkit.dep.module.organization.value}"
          |  def typelevelToolkitDefaultVersion = "${Deps.typelevelToolkitVersion}"
          |  def typelevelToolkitMaxScalaNative = "${Deps.Versions.maxScalaNativeForTypelevelToolkit}"

modules/build/src/main/scala/scala/build/internal/Runner.scala

@@ -189,6 +189,60 @@ object Runner {
       run(command, logger, cwd = cwd, extraEnv = extraEnv)
   }
 
+  // Detects the major version of Node.js on PATH; cached for the JVM lifetime (lazy val).
+  // Returns None if node is not found or version cannot be parsed.
+  private lazy val nodeMajorVersion: Option[Int] =
+    try {
+      val process = new ProcessBuilder("node", "--version")
+        .redirectErrorStream(true)
+        .start()
+      val output = new String(process.getInputStream.readAllBytes()).trim
+      process.waitFor()
+      // Node version format: "v22.5.0" -> extract 22
+      if (output.startsWith("v"))
+        output.drop(1).takeWhile(_.isDigit) match {
+          case s if s.nonEmpty => Some(s.toInt)
+          case _               => None
+        }
+      else None
+    }
+    catch {
+      case _: Exception => None
+    }
+
+  // Node 24+ (V8 13+) has wasm-exnref enabled by default; older versions need --experimental-wasm-exnref.
+  private def nodeNeedsWasmFlag: Boolean =
+    nodeMajorVersion.forall(_ < 24) // true if unknown or < 24
+
+  // Detects the major version of Deno on PATH; cached for the JVM lifetime (lazy val).
+  // Returns None if deno is not found or version cannot be parsed.
+  private lazy val denoMajorVersion: Option[Int] =
+    try {
+      val process = new ProcessBuilder("deno", "--version")
+        .redirectErrorStream(true)
+        .start()
+      val output = new String(process.getInputStream.readAllBytes()).trim
+      process.waitFor()
+      // Deno version format: "deno 2.1.0 (release, aarch64-apple-darwin)\nv8 13.x\ntypescript 5.x"
+      // Extract major from first line
+      val firstLine  = output.linesIterator.nextOption().getOrElse("")
+      val versionStr = firstLine.stripPrefix("deno ").takeWhile(c => c.isDigit || c == '.')
+      versionStr.takeWhile(_.isDigit) match {
+        case s if s.nonEmpty => Some(s.toInt)
+        case _               => None
+      }
+    }
+    catch {
+      case _: Exception => None
+    }
+
+  // Deno 2.x+ bundles V8 13+ which has wasm-exnref enabled by default; no flag needed.
+  private def denoNeedsWasmFlag: Boolean =
+    denoMajorVersion.flatMap { major =>
+      if (major >= 2) Some(false) // Deno 2.x+ has V8 13+ with wasm-exnref by default
+      else Some(true)
+    }.getOrElse(true) // true if unknown
+
   private def endsWithCaseInsensitive(s: String, suffix: String): Boolean =
     s.length >= suffix.length &&
     s.regionMatches(true, s.length - suffix.length, suffix, 0, suffix.length)
@@ -221,11 +275,13 @@ object Runner {
   def jsCommand(
     entrypoint: File,
     args: Seq[String],
-    jsDom: Boolean = false
+    jsDom: Boolean = false,
+    emitWasm: Boolean = false
   ): Seq[String] = {
 
-    val nodePath = findInPath("node").fold("node")(_.toString)
-    val command  = Seq(nodePath, entrypoint.getAbsolutePath) ++ args
+    val nodePath  = findInPath("node").fold("node")(_.toString)
+    val nodeFlags = if (emitWasm && nodeNeedsWasmFlag) List("--experimental-wasm-exnref") else Nil
+    val command   = Seq(nodePath) ++ nodeFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
 
     if (jsDom)
       // FIXME We'd need to replicate what JSDOMNodeJSEnv does under-the-hood to get the command in that case.
@@ -242,14 +298,16 @@ object Runner {
     allowExecve: Boolean = false,
     jsDom: Boolean = false,
     sourceMap: Boolean = false,
-    esModule: Boolean = false
+    esModule: Boolean = false,
+    emitWasm: Boolean = false
   ): Either[BuildException, Process] = either {
     val nodePath: String =
       value(findInPath("node")
         .map(_.toString)
         .toRight(NodeNotFoundError()))
+    val nodeFlags = if (emitWasm && nodeNeedsWasmFlag) List("--experimental-wasm-exnref") else Nil
     if !jsDom && allowExecve && Execve.available() then {
-      val command = Seq(nodePath, entrypoint.getAbsolutePath) ++ args
+      val command = Seq(nodePath) ++ nodeFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
 
       logger.log(
         s"Running ${command.mkString(" ")}",
@@ -265,12 +323,25 @@ object Runner {
       )
       sys.error("should not happen")
     }
+    else if (emitWasm) {
+      // For WASM mode with ES modules, run node directly instead of NodeJSEnv.
+      // NodeJSEnv's stdin piping with "-" doesn't work with Input.ESModule.
+      val command = Seq(nodePath) ++ nodeFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
+
+      logger.log(
+        s"Running ${command.mkString(" ")}",
+        "  Running" + System.lineSeparator() +
+          command.iterator.map(_ + System.lineSeparator()).mkString
+      )
+
+      new ProcessBuilder(command: _*).inheritIO().start()
+    }
     else {
       val nodeArgs =
         // Scala.js runs apps by piping JS to node.
         // If we need to pass arguments, we must first make the piped input explicit
         // with "-", and we pass the user's arguments after that.
-        if args.isEmpty then Nil else "-" :: args.toList
+        nodeFlags ++ (if args.isEmpty then Nil else "-" :: args.toList)
       val envJs =
         if jsDom then
           new JSDOMNodeJSEnv(
@@ -307,6 +378,69 @@ object Runner {
     }
   }
 
+  def denoCommand(
+    entrypoint: File,
+    args: Seq[String],
+    denoPathOpt: Option[String] = None
+  ): Seq[String] = {
+    val denoPath  = denoPathOpt.getOrElse(findInPath("deno").fold("deno")(_.toString))
+    val denoFlags = Seq("run", "--allow-read")
+    Seq(denoPath) ++ denoFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
+  }
+
+  def runDeno(
+    entrypoint: File,
+    args: Seq[String],
+    logger: Logger,
+    allowExecve: Boolean = false,
+    emitWasm: Boolean = false,
+    denoPathOpt: Option[String] = None
+  ): Either[BuildException, Process] = either {
+    val denoPath: String = denoPathOpt.getOrElse {
+      value(findInPath("deno")
+        .map(_.toString)
+        .toRight(DenoNotFoundError()))
+    }
+    val denoFlags = Seq("run", "--allow-read")
+    val extraEnv  =
+      if (emitWasm && denoNeedsWasmFlag) Map("DENO_V8_FLAGS" -> "--experimental-wasm-exnref")
+      else Map.empty
+
+    if (allowExecve && Execve.available()) {
+      val command = Seq(denoPath) ++ denoFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
+
+      logger.log(
+        s"Running ${command.mkString(" ")}",
+        "  Running" + System.lineSeparator() +
+          command.iterator.map(_ + System.lineSeparator()).mkString
+      )
+
+      logger.debug("execve available")
+      Execve.execve(
+        command.head,
+        "deno" +: command.tail.toArray,
+        (sys.env ++ extraEnv).toArray.sorted.map { case (k, v) => s"$k=$v" }
+      )
+      sys.error("should not happen")
+    }
+    else {
+      val command = Seq(denoPath) ++ denoFlags ++ Seq(entrypoint.getAbsolutePath) ++ args
+
+      logger.log(
+        s"Running ${command.mkString(" ")}",
+        "  Running" + System.lineSeparator() +
+          command.iterator.map(_ + System.lineSeparator()).mkString
+      )
+
+      val builder = new ProcessBuilder(command*)
+        .inheritIO()
+      val env = builder.environment()
+      for ((k, v) <- extraEnv)
+        env.put(k, v)
+      builder.start()
+    }
+  }
+
   def runNative(
     launcher: File,
     args: Seq[String],

modules/build/src/main/scala/scala/build/preprocessing/directives/DirectivesPreprocessingUtils.scala

@@ -32,7 +32,8 @@ object DirectivesPreprocessingUtils {
       directives.ScalaVersion.handler,
       directives.Sources.handler,
       directives.Watching.handler,
-      directives.Tests.handler
+      directives.Tests.handler,
+      directives.Wasm.handler
     ).map(_.mapE(_.buildOptions))
 
   val usingDirectiveWithReqsHandlers

modules/cli/src/main/scala/scala/cli/commands/fix/BuiltInRules.scala

@@ -387,6 +387,7 @@ object BuiltInRules extends CommandHelpers {
         JavaHome.handler.keys,
         ScalaNative.handler.keys,
         ScalaJs.handler.keys,
+        Wasm.handler.keys,
         ScalacOptions.handler.keys,
         JavaOptions.handler.keys,
         JavacOptions.handler.keys,