SIL: Don't skip lowering obsolete decls.

When `-unavailable-decl-optimization=complete` is specified obsolete decls
should be preserved because their symbols are still ABI since they are
available to use when targeting deployment targets earlier than the obsoletion
version.

Resolves rdar://110268649

Author: tshortli

lib/SIL/IR/SILModule.cpp

@@ -956,14 +956,21 @@ bool Lowering::usesObjCAllocator(ClassDecl *theClass) {
   return theClass->getObjectModel() == ReferenceCounting::ObjC;
 }
 
+static bool isUnconditionallyUnavailable(const Decl *D) {
+  if (auto unavailableAttrAndDecl = D->getSemanticUnavailableAttr())
+    return unavailableAttrAndDecl->first->isUnconditionallyUnavailable();
+
+  return false;
+}
+
 bool Lowering::shouldSkipLowering(const Decl *D) {
   if (D->getASTContext().LangOpts.UnavailableDeclOptimizationMode !=
       UnavailableDeclOptimization::Complete)
     return false;
 
   // Unavailable declarations should be skipped if
   // -unavailable-decl-optimization=complete is specified.
-  return D->getSemanticUnavailableAttr() != None;
+  return isUnconditionallyUnavailable(D);
 }
 
 bool Lowering::shouldLowerToUnavailableCodeStub(const Decl *D) {
@@ -973,5 +980,5 @@ bool Lowering::shouldLowerToUnavailableCodeStub(const Decl *D) {
 
   // Unavailable declarations should trap at runtime if
   // -unavailable-decl-optimization=stub is specified.
-  return D->getSemanticUnavailableAttr() != None;
+  return isUnconditionallyUnavailable(D);
 }

test/SILGen/unavailable_decl_optimization_complete.swift

@@ -1,5 +1,5 @@
-// RUN: %target-swift-emit-silgen -module-name Test -parse-as-library %s -verify | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
-// RUN: %target-swift-emit-silgen -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=none | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
+// RUN: %target-swift-emit-silgen -swift-version 5 -module-name Test -parse-as-library %s -verify | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
+// RUN: %target-swift-emit-silgen -swift-version 5 -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=none | %FileCheck %s --check-prefixes=CHECK,CHECK-NO-STRIP
 // RUN: %target-swift-emit-silgen -module-name Test -parse-as-library %s -verify -unavailable-decl-optimization=complete | %FileCheck %s --check-prefixes=CHECK,CHECK-STRIP
 
 // CHECK-NO-STRIP: s4Test14globalConstant_Wz
@@ -18,6 +18,10 @@ public let globalConstant = true
 @available(*, unavailable)
 public func unavailableFunc() {}
 
+// CHECK: s4Test21funcObsoletedInSwift5yyF
+@available(swift, introduced: 4.2, obsoleted: 5)
+public func funcObsoletedInSwift5() {}
+
 @available(*, unavailable)
 public struct UnavailableStruct<T> {
   // CHECK-NO-STRIP: s4Test17UnavailableStructV8propertyxvg