Introduce -application-extension-library (#63769)

The current implementation of `-application-extension` has a problem that affects the generation of ObjC headers for regular Swift modules.

The primary purpose of `-application-extension` is to prevent the use of unavailable APIs in app extensions. However, it has an impact on the generation of -Swift.h headers and exposes Swift's internal declarations to ObjC. This behavior is appropriate for mixed modules that are not consumed externally, such as app extensions, but it fails to address the situation when a module is not an extension itself but is consumed by the extension (c90cd11).

To resolve this issue while maintaining the desired behavior, we can introduce a new flag for this particular use-case.

Author: maxovtsin

include/swift/Basic/LangOptions.h

@@ -219,6 +219,9 @@ namespace swift {
     /// Enable 'availability' restrictions for App Extensions.
     bool EnableAppExtensionRestrictions = false;
 
+    /// Enable 'availability' restrictions for App Extension Libraries.
+    bool EnableAppExtensionLibraryRestrictions = false;
+
     /// Diagnostic level to report when a public declarations doesn't declare
     /// an introduction OS version.
     llvm::Optional<DiagnosticBehavior> RequireExplicitAvailability = llvm::None;

include/swift/Option/Options.td

@@ -865,6 +865,10 @@ def enable_app_extension : Flag<["-"], "application-extension">,
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Restrict code to those available for App Extensions">;
 
+def enable_app_extension_library : Flag<["-"], "application-extension-library">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Restrict code to those available for App Extension Libraries">;
+
 def libc : Separate<["-"], "libc">,
            Flags<[SwiftAPIExtractOption, SwiftSymbolGraphExtractOption]>,
            HelpText<"libc runtime library to use">;

lib/AST/Module.cpp

@@ -2715,7 +2715,9 @@ bool ModuleDecl::isExternallyConsumed() const {
   // App extensions are special beasts because they build without entrypoints
   // like library targets, but they behave like executable targets because
   // their associated modules are not suitable for distribution.
-  if (getASTContext().LangOpts.EnableAppExtensionRestrictions) {
+  // However, app extension libraries might be consumed externally.
+  if (getASTContext().LangOpts.EnableAppExtensionRestrictions &&
+      !getASTContext().LangOpts.EnableAppExtensionLibraryRestrictions) {
     return false;
   }
 

lib/Driver/ToolChains.cpp

@@ -251,6 +251,7 @@ void ToolChain::addCommonFrontendArgs(const OutputInfo &OI,
   inputArgs.AddLastArg(arguments, options::OPT_warn_implicit_overrides);
   inputArgs.AddLastArg(arguments, options::OPT_typo_correction_limit);
   inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension);
+  inputArgs.AddLastArg(arguments, options::OPT_enable_app_extension_library);
   inputArgs.AddLastArg(arguments, options::OPT_enable_library_evolution);
   inputArgs.AddLastArg(arguments, options::OPT_require_explicit_availability);
   inputArgs.AddLastArg(arguments, options::OPT_require_explicit_availability_target);

lib/Frontend/CompilerInvocation.cpp

@@ -895,7 +895,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
   Opts.Features.insert(Feature::LayoutPrespecialization);
 
+  Opts.EnableAppExtensionLibraryRestrictions |= Args.hasArg(OPT_enable_app_extension_library);
   Opts.EnableAppExtensionRestrictions |= Args.hasArg(OPT_enable_app_extension);
+  Opts.EnableAppExtensionRestrictions |= Opts.EnableAppExtensionLibraryRestrictions;
 
   Opts.EnableSwift3ObjCInference =
     Args.hasFlag(OPT_enable_swift3_objc_inference,

test/ClangImporter/availability_app_extension.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -application-extension %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -application-extension-library %s
 
 // Check the exact error message, which requires a regex match
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -application-extension %s 2>&1 | %FileCheck %s

test/ClangImporter/availability_ios.swift

@@ -1,5 +1,6 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules %s
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -application-extension %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -application-extension-library %s
 
 // REQUIRES: OS=ios
 

test/ClangImporter/availability_macosx.swift

@@ -1,5 +1,6 @@
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules %s
 // RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -application-extension %s
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -verify -I %S/Inputs/custom-modules -application-extension-library %s
 
 // REQUIRES: OS=macosx
 

test/PrintAsObjC/accessibility.swift

@@ -19,6 +19,10 @@
 // RUN: %FileCheck -check-prefix=CHECK -check-prefix=CHECK-INTERNAL %s < %t/accessibility-appext.h
 // RUN: %check-in-clang %t/accessibility-appext.h
 
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -parse-as-library %s -typecheck -application-extension-library -emit-objc-header-path %t/accessibility-appextlib.h -disable-objc-attr-requires-foundation-module
+// RUN: %FileCheck -check-prefix=CHECK -check-prefix=CHECK-PUBLIC %s < %t/accessibility-appextlib.h
+// RUN: %check-in-clang %t/accessibility-appextlib.h
+
 // REQUIRES: objc_interop
 
 // CHECK: #ifndef ACCESSIBILITY_SWIFT_H

test/attr/attr_availability_transitive_ios_appext.swift

@@ -1,4 +1,5 @@
 // RUN: %target-typecheck-verify-swift -application-extension
+// RUN: %target-typecheck-verify-swift -application-extension-library
 // REQUIRES: OS=ios
 
 // Allow referencing unavailable API in situations where the caller is marked unavailable in the same circumstances.