[Macros] Gate closure body macros behind an experimental feature flag.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -7833,6 +7833,9 @@ ERROR(conformance_macro,none,
 ERROR(experimental_macro,none,
       "macro %0 is experimental",
       (DeclName))
+ERROR(experimental_closure_body_macro,none,
+      "function body macros on closures is experimental",
+      ())
 
 ERROR(macro_resolve_circular_reference, none,
       "circular reference resolving %select{freestanding|attached}0 macro %1",

include/swift/Basic/Features.def

@@ -514,6 +514,9 @@ EXPERIMENTAL_FEATURE(ConcurrencySyntaxSugar, true)
 /// Allow declaration of compile-time values
 EXPERIMENTAL_FEATURE(CompileTimeValues, true)
 
+/// Allow function body macros applied to closures.
+EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -359,6 +359,10 @@ static bool usesFeatureCompileTimeValues(Decl *decl) {
   return decl->getAttrs().hasAttribute<ConstValAttr>();
 }
 
+static bool usesFeatureClosureBodyMacro(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureMemorySafetyAttributes(Decl *decl) {
   if (decl->getAttrs().hasAttribute<SafeAttr>() ||
       decl->getAttrs().hasAttribute<UnsafeAttr>())

lib/Sema/TypeCheckAttr.cpp

@@ -8281,6 +8281,12 @@ class ClosureAttributeChecker
     auto *decl = declRef.getDecl();
     if (auto *macro = dyn_cast_or_null<MacroDecl>(decl)) {
       if (macro->getMacroRoles().contains(MacroRole::Body)) {
+        if (!ctx.LangOpts.hasFeature(Feature::ClosureBodyMacro)) {
+          ctx.Diags.diagnose(
+              attr->getLocation(),
+              diag::experimental_closure_body_macro);
+        }
+
         // Function body macros are allowed on closures.
         return;
       }

test/Macros/start_task.swift

@@ -1,6 +1,6 @@
-// REQUIRES: swift_swift_parser, swift_feature_ConcurrencySyntaxSugar
+// REQUIRES: swift_swift_parser, swift_feature_ConcurrencySyntaxSugar, swift_feature_ClosureBodyMacro
 
-// 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
+// RUN: %target-swift-frontend -typecheck -plugin-path %swift-plugin-dir -enable-experimental-feature ConcurrencySyntaxSugar -enable-experimental-feature ClosureBodyMacro -language-mode 6 %s -dump-macro-expansions 2>&1 | %FileCheck %s
 
 func f() async {}