Introduce a separate feature to cover @freestanding(expression)

Author: DougGregor

include/swift/Basic/Features.def

@@ -92,6 +92,9 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(PrimaryAssociatedTypes2, 346, "Primary associated
 SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
 LANGUAGE_FEATURE(BuiltinIntLiteralAccessors, 368, "Builtin.IntLiteral accessors", true)
+LANGUAGE_FEATURE(
+    FreestandingExpressionMacros, 0, "@freestanding(expression) macros",
+    langOpts.hasFeature(Feature::Macros))
 
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
 UPCOMING_FEATURE(ForwardTrailingClosures, 286, 6)

lib/AST/ASTPrinter.cpp

@@ -3171,6 +3171,14 @@ static bool usesFeatureBuiltinMacros(Decl *decl) {
 
 static bool usesFeatureImportSymbolicCXXDecls(Decl *decl) { return false; }
 
+static bool usesFeatureFreestandingExpressionMacros(Decl *decl) {
+  auto macro = dyn_cast<MacroDecl>(decl);
+  if (!macro)
+    return false;
+
+  return macro->getMacroRoles().contains(MacroRole::Expression);
+}
+
 static void
 suppressingFeatureNoAsyncAvailability(PrintOptions &options,
                                       llvm::function_ref<void()> action) {

test/ModuleInterface/macros.swift

@@ -6,12 +6,12 @@
 // RUN: %FileCheck %s < %t/Macros.swiftinterface --check-prefix CHECK
 // RUN: %target-swift-frontend -compile-module-from-interface %t/Macros.swiftinterface -o %t/Macros.swiftmodule
 
-// CHECK: #if compiler(>=5.3) && $Macros
+// CHECK: #if compiler(>=5.3) && $FreestandingExpressionMacros && $Macros
 // CHECK-NEXT: @freestanding(expression) public macro publicStringify<T>(_ value: T) -> (T, Swift.String) = #externalMacro(module: "SomeModule", type: "StringifyMacro")
 // CHECK-NEXT: #endif
 @freestanding(expression) public macro publicStringify<T>(_ value: T) -> (T, String) = #externalMacro(module: "SomeModule", type: "StringifyMacro")
 
-// CHECK: #if compiler(>=5.3) && $Macros
+// CHECK: #if compiler(>=5.3) && $FreestandingExpressionMacros && $Macros
 // CHECK: @freestanding(expression) public macro publicLine<T>: T = #externalMacro(module: "SomeModule", type: "Line") where T : Swift.ExpressibleByIntegerLiteral
 // CHECK-NEXT: #endif
 @freestanding(expression) public macro publicLine<T: ExpressibleByIntegerLiteral>: T = #externalMacro(module: "SomeModule", type: "Line")