[AST] Protect declarations with @_inheritActorContext(always) by a feature flag

This is going to avoid condfails when declarations are printed
in the swiftinterface files.

Author: xedin

include/swift/Basic/Features.def

@@ -259,6 +259,7 @@ LANGUAGE_FEATURE(IsolatedConformances, 407, "Global-actor isolated conformances"
 LANGUAGE_FEATURE(ValueGenericsNameLookup, 452, "Value generics appearing as static members for namelookup")
 LANGUAGE_FEATURE(GeneralizedIsSameMetaTypeBuiltin, 465, "Builtin.is_same_metatype with support for noncopyable/nonescapable types")
 SUPPRESSIBLE_LANGUAGE_FEATURE(ABIAttributeSE0479, 479, "@abi attribute on functions, initializers, properties, and subscripts")
+LANGUAGE_FEATURE(AlwaysInheritActorContext, 472, "@_inheritActorContext(always)")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

lib/AST/FeatureSet.cpp

@@ -624,6 +624,21 @@ static bool usesFeatureExtensibleAttribute(Decl *decl) {
   return decl->getAttrs().hasAttribute<ExtensibleAttr>();
 }
 
+static bool usesFeatureAlwaysInheritActorContext(Decl *decl) {
+  auto *VD = dyn_cast<ValueDecl>(decl);
+  if (!VD)
+    return false;
+
+  if (auto *PL = VD->getParameterList()) {
+    return llvm::any_of(*PL, [&](const ParamDecl *P) {
+      auto *attr = P->getAttrs().getAttribute<InheritActorContextAttr>();
+      return attr && attr->isAlways();
+    });
+  }
+
+  return false;
+}
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------

test/ModuleInterface/inheritActorContext_attr.swift

@@ -0,0 +1,43 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-emit-module-interface(%t/Library.swiftinterface) %s -module-name Library
+// RUN: %target-swift-typecheck-module-from-interface(%t/Library.swiftinterface) -module-name Library
+// RUN: %FileCheck %s < %t/Library.swiftinterface
+
+// CHECK-NOT: #if compiler(>=5.3) && $AlwaysInheritActorContext
+// CHECK: public func globalTest(@_inheritActorContext _: @Sendable () async -> Swift.Void)
+// CHECK-NOT: #endif
+public func globalTest(@_inheritActorContext _: @Sendable () async -> Void) {}
+
+// CHECK: #if compiler(>=5.3) && $AlwaysInheritActorContext
+// CHECK-NEXT: public func globalTestAlways(@_inheritActorContext(always) _: @Sendable () async -> Swift.Void)
+// CHECK-NEXT: #endif
+public func globalTestAlways(@_inheritActorContext(always) _: @Sendable () async -> Void) {}
+
+public struct Test {
+  // CHECK-NOT: #if compiler(>=5.3) && $AlwaysInheritActorContext
+  // CHECK: public init(@_inheritActorContext x: @Sendable () async -> Swift.Int)
+  // CHECK-NOT: #endif
+  public init(@_inheritActorContext x: @Sendable () async -> Int) {}
+
+  // CHECK: #if compiler(>=5.3) && $AlwaysInheritActorContext
+  // CHECK-NEXT: #if compiler(>=5.3) && $SendingArgsAndResults
+  // CHECK-NEXT: public init(@_inheritActorContext(always) y: sending () async -> Swift.Void)
+  // CHECK-NEXT: #else
+  // CHECK-NEXT: public init(@_inheritActorContext(always) y: () async -> Swift.Void)
+  // CHECK-NEXT: #endif
+  // CHECK-NEXT: #endif
+  public init(@_inheritActorContext(always) y: sending () async -> Void) {}
+
+  // CHECK-NOT: #if compiler(>=5.3) && $AlwaysInheritActorContext
+  // CHECK: public subscript(@_inheritActorContext _: @Sendable () async -> Swift.Void) -> Swift.Bool {
+  // CHECK-NEXT:   get
+  // CHECK-NEXT: }
+  // CHECK-NOT: #endif
+  public subscript(@_inheritActorContext _: @Sendable () async -> Void) -> Bool { false }
+
+  // CHECK: #if compiler(>=5.3) && $AlwaysInheritActorContext
+  // CHECK-NEXT: public subscript(@_inheritActorContext(always) _: @Sendable (Swift.Int) async -> Swift.Void) -> Swift.Bool {
+  // CHECK-NEXT:   get
+  // CHECK-NEXT: }
+  public subscript(@_inheritActorContext(always) _: @Sendable (Int) async -> Void) -> Bool { false }
+}