[Concurrency] Add an experimental macro for wrapping a function body in a new task.

include/swift/AST/DiagnosticsSema.def

@@ -7810,6 +7810,10 @@ ERROR(conformance_macro,none,
       "conformance macros are replaced by extension macros",
       ())
 
+ERROR(experimental_macro,none,
+      "macro %0 is experimental",
+      (DeclName))
+
 ERROR(macro_resolve_circular_reference, none,
       "circular reference resolving %select{freestanding|attached}0 macro %1",
       (bool, DeclName))

include/swift/Basic/Features.def

@@ -473,6 +473,9 @@ EXPERIMENTAL_FEATURE(ExtensibleEnums, true)
 /// Allow isolated conformances.
 EXPERIMENTAL_FEATURE(IsolatedConformances, true)
 
+/// Syntax sugar features for concurrency.
+EXPERIMENTAL_FEATURE(ConcurrencySyntaxSugar, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -350,6 +350,10 @@ static bool usesFeatureIsolatedConformances(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureConcurrencySyntaxSugar(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureMemorySafetyAttributes(Decl *decl) {
   if (decl->getAttrs().hasAttribute<SafeAttr>() ||
       decl->getAttrs().hasAttribute<UnsafeAttr>())

lib/Macros/Sources/SwiftMacros/CMakeLists.txt

@@ -14,6 +14,7 @@ add_swift_macro_library(SwiftMacros
   OptionSetMacro.swift
   DebugDescriptionMacro.swift
   DistributedResolvableMacro.swift
+  StartTaskMacro.swift
   SyntaxExtensions.swift
   TaskLocalMacro.swift
   SwiftifyImportMacro.swift

lib/Macros/Sources/SwiftMacros/StartTaskMacro.swift

@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftDiagnostics
+import SwiftParser
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+
+struct TaskMacroDiagnostic: DiagnosticMessage {
+  static func diagnose(at node: some SyntaxProtocol) -> Diagnostic {
+    Diagnostic(node: Syntax(node), message: Self.init())
+  }
+
+  var message: String {
+    "'@StartTask' macro can only be used on functions with an implementation"
+  }
+
+  var severity: DiagnosticSeverity { .error }
+
+  var diagnosticID: MessageID {
+    MessageID(domain: "_Concurrency", id: "StartMacro.\(self)")
+  }
+}
+
+
+public struct StartTaskMacro: BodyMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingBodyFor declaration: some DeclSyntaxProtocol & WithOptionalCodeBlockSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [CodeBlockItemSyntax] {
+    guard let taskBody = declaration.body else {
+      context.diagnose(TaskMacroDiagnostic.diagnose(at: node))
+      return []
+    }
+
+    return [
+      """
+      Task \(taskBody)
+      """
+    ]
+  }
+}

lib/Sema/TypeCheckMacros.cpp

@@ -1735,6 +1735,19 @@ ExpandBodyMacroRequest::evaluate(Evaluator &evaluator,
         if (bufferID)
           return;
 
+        // '@StartTask' is gated behind the 'ConcurrencySyntaxSugar'
+        // experimental feature.
+        auto &ctx = fn->getASTContext();
+        if (macro->getParentModule()->getName().is("_Concurrency") &&
+            macro->getBaseIdentifier().is("StartTask") &&
+            !ctx.LangOpts.hasFeature(Feature::ConcurrencySyntaxSugar)) {
+          ctx.Diags.diagnose(
+              customAttr->getLocation(),
+              diag::experimental_macro,
+              macro->getName());
+          return;
+        }
+
         auto macroSourceFile = ::evaluateAttachedMacro(
             macro, fn, customAttr, false, MacroRole::Body);
         if (!macroSourceFile)

stdlib/public/Concurrency/Actor.swift

@@ -98,6 +98,11 @@ internal func _enqueueOnMain(_ job: UnownedJob)
 @available(SwiftStdlib 5.1, *)
 @freestanding(expression)
 public macro isolation<T>() -> T = Builtin.IsolationMacro
+
+@available(SwiftStdlib 5.1, *)
+@attached(body)
+public macro StartTask() =
+  #externalMacro(module: "SwiftMacros", type: "StartTaskMacro")
 #endif
 
 #if $IsolatedAny

test/Macros/start_task.swift

@@ -0,0 +1,18 @@
+// REQUIRES: swift_swift_parser, swift_feature_ConcurrencySyntaxSugar
+
+// RUN: %target-swift-frontend -typecheck -plugin-path %swift-plugin-dir -enable-experimental-feature ConcurrencySyntaxSugar -language-mode 6 %s -dump-macro-expansions 2>&1 | %FileCheck %s
+
+func f() async {}
+
+// CHECK-LABEL: @__swiftmacro_10start_task4sync9StartTaskfMb_.swift
+// CHECK: Task {
+// CHECK:   await f()
+// CHECK: }
+
+@StartTask
+func sync() {
+  await f()
+}
+
+
+