IRGen: Weakly link symbols for unavailable declarations.

When computing linkage, the compiler would treat unavailable declarations as if
they were "always available" when they lack an `introduced:` version:

```
// Library
@available(macOS, unavailable)
public func foo() {
  // …
}

// Client
import Library

@available(macOS, unavailable)
func bar() {
  // Even though foo() and bar() are unavalable on macOS the compiler still
  // strongly links foo().
  foo()
}
```

This created an unnecessary dependency between libraries and their clients and
also can interfere with back deployment, since unavailable declarations may not
be present in a library on all OS versions. Developers could work around these
issues by conditionally compiling the code that references an unavailable
declaration, but they shouldn't have to given that unavailable code is meant to
be provably unreachable at runtime. Additionally, it could improve library code
size if we allowed the compiler to strip unavailable declarations from a binary
completely.

Resolves rdar://106673713

Author: tshortli

lib/AST/Decl.cpp

@@ -1126,6 +1126,9 @@ bool Decl::isAlwaysWeakImported() const {
   if (getAttrs().hasAttribute<WeakLinkedAttr>())
     return true;
 
+  if (getSemanticUnavailableAttr())
+    return true;
+
   if (auto *accessor = dyn_cast<AccessorDecl>(this))
     return accessor->getStorage()->isAlwaysWeakImported();
 

test/IRGen/Inputs/weak_import_availability_helper.swift

@@ -1,27 +1,51 @@
 @available(macOS 10.50, *)
 public func conditionallyAvailableFunction() {}
 
+@available(macOS, unavailable)
+public func unavailableFunction() {}
+
 @available(macOS 10.50, *)
 public var conditionallyAvailableGlobal: Int {
   get {return 0}
   set {}
 }
 
+@available(macOS, unavailable)
+public var unavailableGlobal: Int {
+  get {return 0}
+  set {}
+}
+
 @available(macOS 10.50, *)
 public struct ConditionallyAvailableStruct {
   public func conditionallyAvailableMethod() {}
 }
 
+extension ConditionallyAvailableStruct {
+  public struct NestedStruct {}
+}
+
+@available(macOS, unavailable)
+public struct UnvailableStruct {
+  public func unavailableMethod() {}
+}
+
 public protocol AlwaysAvailableProtocol {}
 
 public struct AlwaysAvailableStruct {}
 
 @available(macOS 10.50, *)
 extension AlwaysAvailableStruct : AlwaysAvailableProtocol {}
 
+@available(macOS, unavailable)
+public protocol UnavailableProtocol {}
+
+@available(macOS, unavailable)
+extension AlwaysAvailableStruct : UnavailableProtocol {}
+
 public enum AlwaysAvailableEnum {
   case alwaysAvailableCase
 
   @available(macOS 10.50, *)
   case conditionallyAvailableCase
-}
+}

test/IRGen/weak_import_availability.swift

@@ -1,12 +1,12 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -emit-module-path %t/weak_import_availability_helper.swiftmodule -parse-as-library %S/Inputs/weak_import_availability_helper.swift -enable-library-evolution
+// RUN: %target-swift-frontend -target %target-cpu-apple-macosx10.50 -emit-module -emit-module-path %t/weak_import_availability_helper.swiftmodule -parse-as-library %S/Inputs/weak_import_availability_helper.swift -enable-library-evolution
 //
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir | %FileCheck %s --check-prefix=CHECK-OLD
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 | %FileCheck %s --check-prefix=CHECK-NEW
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 | %FileCheck %s --check-prefix=CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir | %FileCheck %s --check-prefixes=CHECK,CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
 
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 -weak-link-at-target | %FileCheck %s --check-prefix=CHECK-OLD
-// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 -weak-link-at-target | %FileCheck %s --check-prefix=CHECK-NEW
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.50 -weak-link-at-target | %FileCheck %s --check-prefixes=CHECK,CHECK-OLD
+// RUN: %target-swift-frontend -primary-file %s -I %t -emit-ir -target %target-cpu-apple-macosx10.60 -weak-link-at-target | %FileCheck %s --check-prefixes=CHECK,CHECK-NEW
 
 // REQUIRES: OS=macosx
 
@@ -35,6 +35,16 @@ public func useConditionallyAvailableConformance() {
 // CHECK-OLD-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA0eF8ProtocolAAWP" = extern_weak global i8*
 // CHECK-NEW-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA0eF8ProtocolAAWP" = external global i8*
 
+@available(macOS, unavailable)
+func useUnavailableConformance<T : UnavailableProtocol>(_: T.Type) {}
+
+@available(macOS, unavailable)
+public func useUnavailableConformance() {
+  useUnavailableConformance(AlwaysAvailableStruct.self)
+}
+
+// CHECK-LABEL: @"$s31weak_import_availability_helper21AlwaysAvailableStructVAA19UnavailableProtocolAAWP" = extern_weak global i8*, align 8
+
 @available(macOS 10.50, *)
 public func callConditionallyAvailableFunction() {
   conditionallyAvailableFunction()
@@ -43,6 +53,13 @@ public func callConditionallyAvailableFunction() {
 // CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper30conditionallyAvailableFunctionyyF"()
 // CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper30conditionallyAvailableFunctionyyF"()
 
+@available(macOS, unavailable)
+public func callUnavailableFunction() {
+  unavailableFunction()
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper19unavailableFunctionyyF"()
+
 @available(macOS 10.50, *)
 public func useConditionallyAvailableGlobal() {
   _ = conditionallyAvailableGlobal
@@ -59,6 +76,17 @@ public func useConditionallyAvailableGlobal() {
 // CHECK-OLD-LABEL: declare extern_weak swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivM"(i8* noalias dereferenceable(32))
 // CHECK-NEW-LABEL: declare swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper28conditionallyAvailableGlobalSivM"(i8* noalias dereferenceable(32))
 
+@available(macOS, unavailable)
+public func useUnavailableGlobal() {
+  _ = unavailableGlobal
+  unavailableGlobal = 0
+  unavailableGlobal += 1
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc i64 @"$s31weak_import_availability_helper17unavailableGlobalSivg"()
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper17unavailableGlobalSivs"(i64)
+// CHECK-LABEL: declare extern_weak swiftcc { i8*, %TSi* } @"$s31weak_import_availability_helper17unavailableGlobalSivM"(i8* noalias dereferenceable(32))
+
 func blackHole<T>(_: T) {}
 
 @available(macOS 10.50, *)
@@ -69,10 +97,32 @@ public func useConditionallyAvailableStruct() {
 // CHECK-OLD-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructVMa"(i64)
 // CHECK-NEW-LABEL: declare swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructVMa"(i64)
 
+@available(macOS 10.50, *)
+public func useNestedConditionallyAvailableStruct() {
+  blackHole(ConditionallyAvailableStruct.NestedStruct.self)
+}
+
+// CHECK-OLD-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructV06NestedG0VMa"(i64)
+// CHECK-NEW-LABEL: declare swiftcc %swift.metadata_response @"$s31weak_import_availability_helper28ConditionallyAvailableStructV06NestedG0VMa"(i64)
+
 @available(macOS 10.50, *)
 public func useConditionallyAvailableMethod(s: ConditionallyAvailableStruct) {
   s.conditionallyAvailableMethod()
 }
 
 // CHECK-OLD-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper28ConditionallyAvailableStructV013conditionallyF6MethodyyF"(%swift.opaque* noalias nocapture swiftself)
 // CHECK-NEW-LABEL: declare swiftcc void @"$s31weak_import_availability_helper28ConditionallyAvailableStructV013conditionallyF6MethodyyF"(%swift.opaque* noalias nocapture swiftself)
+
+@available(macOS, unavailable)
+public func useUnavailableStruct() {
+  blackHole(UnvailableStruct.self)
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc %swift.metadata_response @"$s31weak_import_availability_helper16UnvailableStructVMa"(i64)
+
+@available(macOS, unavailable)
+public func useUnavailableMethod(s: UnvailableStruct) {
+  s.unavailableMethod()
+}
+
+// CHECK-LABEL: declare extern_weak swiftcc void @"$s31weak_import_availability_helper16UnvailableStructV17unavailableMethodyyF"(%swift.opaque* noalias nocapture swiftself)

test/SILGen/objc_init_unavailable.swift

@@ -1,12 +1,13 @@
 // RUN: %target-swift-emit-silgen(mock-sdk: %clang-importer-sdk) -enable-objc-interop -import-objc-header %S/Inputs/objc_init_unavailable.h %s | %FileCheck %s
 // REQUIRES: objc_interop
+// REQUIRES: OS=macosx
 
 @available(macOS, unavailable)
 public func callUnavailableInit(name: String) -> ClassWithUnavailableInit {
   return ClassWithUnavailableInit(bundleID: name)
 }
 
-// CHECK-LABEL: sil [ossa] @$s21objc_init_unavailable19callUnavailableInit4nameSo09ClassWitheF0CSS_tF : $@convention(thin) (@guaranteed String) -> @owned ClassWithUnavailableInit {
+// CHECK-LABEL: sil [weak_imported] [ossa] @$s21objc_init_unavailable19callUnavailableInit4nameSo09ClassWitheF0CSS_tF : $@convention(thin) (@guaranteed String) -> @owned ClassWithUnavailableInit {
 // CHECK: function_ref @$sSo24ClassWithUnavailableInitC8bundleIDABSgSSSg_tcfC : $@convention(method) (@owned Optional<String>, @thick ClassWithUnavailableInit.Type) -> @owned Optional<ClassWithUnavailableInit>
 // CHECK: return