Add experimental feature flags for @implementation

• ObjCImplementation controls @implementation on extensions
• CImplementation controls @implementation and @_objcImplementation on cdecl functions

Why the difference between them? Because `@_objcImplementation extension` has already been adopted pretty widely, while `@_objcImplementation @_cdecl` is very new.

Author: beccadax

include/swift/Basic/Features.def

@@ -361,6 +361,12 @@ CONDITIONALLY_SUPPRESSIBLE_EXPERIMENTAL_FEATURE(IsolatedAny, true)
 // Enable usability improvements for global-actor-isolated types.
 EXPERIMENTAL_FEATURE(GlobalActorIsolatedTypesUsability, false)
 
+// Enable @implementation on extensions of ObjC classes.
+EXPERIMENTAL_FEATURE(ObjCImplementation, true)
+
+// Enable @implementation on @_cdecl functions.
+EXPERIMENTAL_FEATURE(CImplementation, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -680,6 +680,9 @@ static bool usesFeatureGlobalActorIsolatedTypesUsability(Decl *decl) {
   return false;
 }
 
+UNINTERESTING_FEATURE(ObjCImplementation)
+UNINTERESTING_FEATURE(CImplementation)
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------

lib/Sema/TypeCheckAttr.cpp

@@ -1516,9 +1516,32 @@ void AttributeChecker::visitNonObjCAttr(NonObjCAttr *attr) {
   }
 }
 
+static bool hasObjCImplementationFeature(Decl *D, ObjCImplementationAttr *attr,
+                                         Feature requiredFeature) {
+  if (D->getASTContext().LangOpts.hasFeature(requiredFeature)) {
+    return true;
+  }
+
+  // Allow the use of @_objcImplementation *without* Feature::ObjCImplementation
+  // as long as you're using the early adopter syntax. (Avoids breaking existing
+  // adopters.)
+  if (requiredFeature == Feature::ObjCImplementation && attr->isEarlyAdopter())
+    return true;
+
+  // Either you're using Feature::ObjCImplementation without the early adopter
+  // syntax, or you're using Feature::CImplementation. Either way, no go.
+  swift::diagnoseAndRemoveAttr(D, attr, diag::requires_experimental_feature,
+                               attr->getAttrName(), attr->isDeclModifier(),
+                               getFeatureName(requiredFeature));
+  return false;
+}
+
 void AttributeChecker::
 visitObjCImplementationAttr(ObjCImplementationAttr *attr) {
   if (auto ED = dyn_cast<ExtensionDecl>(D)) {
+    if (!hasObjCImplementationFeature(D, attr, Feature::ObjCImplementation))
+      return;
+
     if (ED->isConstrainedExtension())
       diagnoseAndRemoveAttr(attr,
                             diag::attr_objc_implementation_must_be_unconditional);
@@ -1575,6 +1598,9 @@ visitObjCImplementationAttr(ObjCImplementationAttr *attr) {
     }
   }
   else if (auto AFD = dyn_cast<AbstractFunctionDecl>(D)) {
+    if (!hasObjCImplementationFeature(D, attr, Feature::CImplementation))
+      return;
+
     if (!attr->CategoryName.empty()) {
       auto diagnostic =
           diagnose(attr->getLocation(),

test/IRGen/objc_implementation.swift

@@ -1,7 +1,7 @@
 // Test doesn't pass on all platforms (rdar://101420862)
 // REQUIRES: OS=macosx
 
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -I %S/Inputs/abi -F %clang-importer-sdk-path/frameworks %s -import-objc-header %S/Inputs/objc_implementation.h -emit-ir > %t.ir
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -enable-experimental-feature CImplementation -I %S/Inputs/abi -F %clang-importer-sdk-path/frameworks %s -import-objc-header %S/Inputs/objc_implementation.h -emit-ir > %t.ir
 // RUN: %FileCheck --input-file %t.ir %s
 // RUN: %FileCheck --input-file %t.ir --check-prefix NEGATIVE %s
 // REQUIRES: objc_interop

test/Interpreter/objc_implementation.swift

@@ -1,4 +1,4 @@
-// RUN: %target-run-simple-swift(-import-objc-header %S/Inputs/objc_implementation.h -D TOP_LEVEL_CODE -swift-version 5) %s | %FileCheck %s
+// RUN: %target-run-simple-swift(-import-objc-header %S/Inputs/objc_implementation.h -D TOP_LEVEL_CODE -swift-version 5 -enable-experimental-feature CImplementation) %s | %FileCheck %s
 // REQUIRES: executable_test
 // REQUIRES: objc_interop
 

test/Interpreter/objc_implementation_swift_client.swift

@@ -12,7 +12,7 @@
 // RUN: %empty-directory(%t/frameworks/objc_implementation.framework/Headers)
 // RUN: cp %S/Inputs/objc_implementation.modulemap %t/frameworks/objc_implementation.framework/Modules/module.modulemap
 // RUN: cp %S/Inputs/objc_implementation.h %t/frameworks/objc_implementation.framework/Headers
-// RUN: %target-build-swift-dylib(%t/frameworks/objc_implementation.framework/objc_implementation) -emit-module-path %t/frameworks/objc_implementation.framework/Modules/objc_implementation.swiftmodule/%module-target-triple.swiftmodule -module-name objc_implementation -F %t/frameworks -import-underlying-module -Xlinker -install_name -Xlinker %t/frameworks/objc_implementation.framework/objc_implementation %S/objc_implementation.swift
+// RUN: %target-build-swift-dylib(%t/frameworks/objc_implementation.framework/objc_implementation) -enable-experimental-feature CImplementation -emit-module-path %t/frameworks/objc_implementation.framework/Modules/objc_implementation.swiftmodule/%module-target-triple.swiftmodule -module-name objc_implementation -F %t/frameworks -import-underlying-module -Xlinker -install_name -Xlinker %t/frameworks/objc_implementation.framework/objc_implementation %S/objc_implementation.swift
 
 //
 // Execute this file

test/PrintAsObjC/objc_implementation.swift

@@ -11,8 +11,8 @@
 // FIXME: END -enable-source-import hackaround
 
 
-// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -emit-module -I %S/Inputs/custom-modules -import-underlying-module -o %t %s -disable-objc-attr-requires-foundation-module
-// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -parse-as-library %t/objc_implementation.swiftmodule -typecheck -I %S/Inputs/custom-modules -emit-objc-header-path %t/objc_implementation-Swift.h -import-underlying-module -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -emit-module -enable-experimental-feature CImplementation -I %S/Inputs/custom-modules -import-underlying-module -o %t %s -disable-objc-attr-requires-foundation-module
+// RUN: %target-swift-frontend(mock-sdk: -sdk %S/../Inputs/clang-importer-sdk -I %t) -parse-as-library %t/objc_implementation.swiftmodule -typecheck -enable-experimental-feature CImplementation -I %S/Inputs/custom-modules -emit-objc-header-path %t/objc_implementation-Swift.h -import-underlying-module -disable-objc-attr-requires-foundation-module
 // RUN: %FileCheck %s --input-file %t/objc_implementation-Swift.h
 // RUN: %FileCheck --check-prefix=NEGATIVE %s --input-file %t/objc_implementation-Swift.h
 // RUN: %check-in-clang -I %S/Inputs/custom-modules/ %t/objc_implementation-Swift.h

test/decl/ext/cdecl_implementation_features.swift

@@ -0,0 +1,11 @@
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h 2>&1 | %FileCheck --check-prefixes NO,CHECK %s
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature CImplementation 2>&1 | %FileCheck --check-prefixes YES,C,CHECK %s
+
+// NO-DAG: cdecl_implementation_features.swift:[[@LINE+3]]:{{[0-9]+}}: error: '_objcImplementation' attribute is only valid when experimental feature CImplementation is enabled
+// YES-NOT: cdecl_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
+// TODO: When @implementation @_cdecl stabilizes, YES-NOT on the line above will become YES-DAG
+@_objcImplementation @_cdecl("CImplFunc1") func CImplFunc1(_: Double) {}
+
+// NO-DAG: cdecl_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature CImplementation is enabled
+// YES-NOT: cdecl_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature CImplementation is enabled
+@implementation @_cdecl("CImplFunc2") func CImplFunc2(_: Double) {}

test/decl/ext/objc_implementation.swift

@@ -1,4 +1,4 @@
-// RUN: %target-typecheck-verify-swift -import-objc-header %S/Inputs/objc_implementation.h
+// RUN: %target-typecheck-verify-swift -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature CImplementation
 // REQUIRES: objc_interop
 
 protocol EmptySwiftProto {}

test/decl/ext/objc_implementation_features.swift

@@ -0,0 +1,10 @@
+// REQUIRES: objc_interop
+
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h 2>&1 | %FileCheck --check-prefixes NO,CHECK %s
+// RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature ObjCImplementation 2>&1 | %FileCheck --check-prefixes YES,CHECK %s
+
+@_objcImplementation extension ObjCSubclass {}
+
+// NO-DAG: objc_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature ObjCImplementation is enabled
+// YES-NOT: objc_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature ObjCImplementation is enabled
+@implementation extension ObjCBasicInitClass {}