AST: Don't optimize decls that are unavailable in extensions.

Unavailable decl optimization is meant to optimize declarations that are
unreachable at runtime. A declaration that is marked unavailable in app
extensions (e.g. `@available(macOSApplicationExtension, unavailable)`) may be
reachable by non-extension processes so it cannot be safely optimized.

Resolves rdar://122924862

Author: tshortli

include/swift/AST/PlatformKind.h

@@ -57,6 +57,12 @@ StringRef prettyPlatformString(PlatformKind platform);
 llvm::Optional<PlatformKind>
 basePlatformForExtensionPlatform(PlatformKind Platform);
 
+/// Returns true if \p Platform represents and application extension platform,
+/// e.g. `iOSApplicationExtension`.
+inline bool isApplicationExtensionPlatform(PlatformKind Platform) {
+  return basePlatformForExtensionPlatform(Platform).has_value();
+}
+
 /// Returns whether the passed-in platform is active, given the language
 /// options. A platform is active if either it is the target platform or its
 /// AppExtension variant is the target platform. For example, OS X is

lib/AST/Availability.cpp

@@ -321,11 +321,29 @@ llvm::Optional<AvailableAttrDeclPair> Decl::getSemanticUnavailableAttr() const {
                            llvm::None);
 }
 
-static bool isUnconditionallyUnavailable(const Decl *D) {
-  if (auto unavailableAttrAndDecl = D->getSemanticUnavailableAttr())
-    return unavailableAttrAndDecl->first->isUnconditionallyUnavailable();
+static bool shouldStubOrSkipUnavailableDecl(const Decl *D) {
+  // Don't trust unavailability on declarations from clang modules.
+  if (isa<ClangModuleUnit>(D->getDeclContext()->getModuleScopeContext()))
+    return false;
 
-  return false;
+  auto unavailableAttrAndDecl = D->getSemanticUnavailableAttr();
+  if (!unavailableAttrAndDecl)
+    return false;
+
+  // getSemanticUnavailableAttr() can return an @available attribute that makes
+  // its declaration unavailable conditionally due to deployment target. Only
+  // stub or skip a declaration that is unavailable regardless of deployment
+  // target.
+  auto *unavailableAttr = unavailableAttrAndDecl->first;
+  if (!unavailableAttr->isUnconditionallyUnavailable())
+    return false;
+
+  // If the decl is only unavailable to app extensions it still may need to be
+  // present at runtime for non-extension processes.
+  if (isApplicationExtensionPlatform(unavailableAttr->Platform))
+    return false;
+
+  return true;
 }
 
 static UnavailableDeclOptimization
@@ -343,10 +361,7 @@ bool Decl::isAvailableDuringLowering() const {
       UnavailableDeclOptimization::Complete)
     return true;
 
-  if (isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext()))
-    return true;
-
-  return !isUnconditionallyUnavailable(this);
+  return !shouldStubOrSkipUnavailableDecl(this);
 }
 
 bool Decl::requiresUnavailableDeclABICompatibilityStubs() const {
@@ -356,10 +371,7 @@ bool Decl::requiresUnavailableDeclABICompatibilityStubs() const {
       UnavailableDeclOptimization::Stub)
     return false;
 
-  if (isa<ClangModuleUnit>(getDeclContext()->getModuleScopeContext()))
-    return false;
-
-  return isUnconditionallyUnavailable(this);
+  return shouldStubOrSkipUnavailableDecl(this);
 }
 
 bool UnavailabilityReason::requiresDeploymentTargetOrEarlier(

lib/AST/PlatformKind.cpp

@@ -109,10 +109,6 @@ swift::basePlatformForExtensionPlatform(PlatformKind Platform) {
   llvm_unreachable("bad PlatformKind");
 }
 
-static bool isApplicationExtensionPlatform(PlatformKind Platform) {
-  return basePlatformForExtensionPlatform(Platform).has_value();
-}
-
 static bool isPlatformActiveForTarget(PlatformKind Platform,
                                       const llvm::Triple &Target,
                                       bool EnableAppExtensionRestrictions) {

test/SILGen/unavailable_decl_optimization_stub_macos.swift

@@ -1,5 +1,7 @@
-// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.8-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub | %FileCheck %s --check-prefixes=CHECK-SWIFT5_8
-// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.9-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub | %FileCheck %s --check-prefixes=CHECK-SWIFT5_9
+// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.8-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub | %FileCheck %s --check-prefixes=CHECK,CHECK-SWIFT5_8
+// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.8-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub -application-extension | %FileCheck %s --check-prefixes=CHECK,CHECK-SWIFT5_8
+// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.9-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub | %FileCheck %s --check-prefixes=CHECK,CHECK-SWIFT5_9
+// RUN: %target-swift-emit-silgen -target %target-swift-abi-5.9-triple -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=stub -application-extension | %FileCheck %s --check-prefixes=CHECK,CHECK-SWIFT5_9
 
 // REQUIRES: OS=macosx
 
@@ -19,3 +21,22 @@ public func unavailableFunc() {}
 @available(*, unavailable)
 @inlinable public func unavailableInlinableFunc() {}
 
+// CHECK-LABEL:     sil{{.*}}@$s4Test22unavailableOnMacOSFuncyyF
+// CHECK-SWIFT5_8:    [[FNREF:%.*]] = function_ref @$ss36_diagnoseUnavailableCodeReached_aeics5NeverOyF : $@convention(thin) () -> Never
+// CHECK-SWIFT5_9:    [[FNREF:%.*]] = function_ref @$ss31_diagnoseUnavailableCodeReacheds5NeverOyF : $@convention(thin) () -> Never
+// CHECK-NEXT:        [[APPLY:%.*]] = apply [[FNREF]]()
+// CHECK:           } // end sil function '$s4Test22unavailableOnMacOSFuncyyF'
+@available(macOS, unavailable)
+public func unavailableOnMacOSFunc() {}
+
+// CHECK-LABEL:     sil{{.*}}@$s4Test31unavailableOnMacOSExtensionFuncyyF
+// CHECK-NOT:         function_ref @$ss36_diagnoseUnavailableCodeReached{{.*}} : $@convention(thin) () -> Never
+// CHECK:           } // end sil function '$s4Test31unavailableOnMacOSExtensionFuncyyF'
+@available(macOSApplicationExtension, unavailable)
+public func unavailableOnMacOSExtensionFunc() {}
+
+// CHECK-LABEL:     sil{{.*}}@$s4Test20unavailableOniOSFuncyyF
+// CHECK-NOT:         function_ref @$ss36_diagnoseUnavailableCodeReached{{.*}} : $@convention(thin) () -> Never
+// CHECK:           } // end sil function '$s4Test20unavailableOniOSFuncyyF'
+@available(iOS, unavailable)
+public func unavailableOniOSFunc() {}