[cxx-interop] Make SWIFT_RETURNS_INDEPENDENT_VALUE imply immortal lifetime

The semantics of returning independent value already matches what
immortal lifetimes are within Swift. This patch makes sure this
annotation works as expected with non-escapable types.

rdar://137671642

Author: Gabor Horvath

lib/ClangImporter/ImportDecl.cpp

@@ -3854,6 +3854,18 @@ namespace {
       if (decl->getTemplatedKind() == clang::FunctionDecl::TK_FunctionTemplate)
         return;
 
+      SmallVector<LifetimeDependenceInfo, 1> lifetimeDependencies;
+      LifetimeDependenceInfo immortalLifetime(nullptr, nullptr, 0,
+                                              /*isImmortal*/ true);
+      if (const auto *funDecl = dyn_cast<FuncDecl>(result))
+        if (hasUnsafeAPIAttr(decl) && !funDecl->getResultInterfaceType()->isEscapable()) {
+          Impl.SwiftContext.evaluator.cacheOutput(
+              LifetimeDependenceInfoRequest{result},
+              Impl.SwiftContext.AllocateCopy(lifetimeDependencies));
+          lifetimeDependencies.push_back(immortalLifetime);
+          return;
+        }
+
       auto retType = decl->getReturnType();
       auto warnForEscapableReturnType = [&] {
         if (isEscapableAnnotatedType(retType.getTypePtr())) {
@@ -3866,8 +3878,8 @@ namespace {
       };
 
       auto swiftParams = result->getParameters();
-      bool hasSelf = result->hasImplicitSelfDecl() && !isa<ConstructorDecl>(result);
-      SmallVector<LifetimeDependenceInfo, 1> lifetimeDependencies;
+      bool hasSelf =
+          result->hasImplicitSelfDecl() && !isa<ConstructorDecl>(result);
       SmallBitVector inheritLifetimeParamIndicesForReturn(swiftParams->size() +
                                                           hasSelf);
       SmallBitVector scopedLifetimeParamIndicesForReturn(swiftParams->size() +
@@ -3907,8 +3919,7 @@ namespace {
         // Assume default constructed view types have no dependencies.
         if (ctordecl->isDefaultConstructor() &&
             importer::hasNonEscapableAttr(ctordecl->getParent()))
-          lifetimeDependencies.push_back(
-              LifetimeDependenceInfo(nullptr, nullptr, 0, /*isImmortal*/ true));
+          lifetimeDependencies.push_back(immortalLifetime);
       }
       if (lifetimeDependencies.empty()) {
         if (isNonEscapableAnnotatedType(retType.getTypePtr())) {

test/Interop/Cxx/class/nonescapable-lifetimebound.swift

@@ -1,6 +1,5 @@
 // RUN: rm -rf %t
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend -typecheck -I %swift_src_root/lib/ClangImporter/SwiftBridging  -I %t/Inputs  %t/test.swift -enable-experimental-feature NonescapableTypes -cxx-interoperability-mode=default -diagnostic-style llvm 2>&1
 // RUN: %target-swift-frontend -I %swift_src_root/lib/ClangImporter/SwiftBridging  -I %t/Inputs -emit-sil %t/test.swift -enable-experimental-feature NonescapableTypes -cxx-interoperability-mode=default -diagnostic-style llvm 2>&1 | %FileCheck %s
 
 //--- Inputs/module.modulemap
@@ -77,6 +76,10 @@ private:
     const int *member;
 };
 
+View returnsImmortal() SWIFT_RETURNS_INDEPENDENT_VALUE {
+    return View();
+}
+
 // CHECK: sil [clang makeOwner] {{.*}}: $@convention(c) () -> Owner
 // CHECK: sil [clang getView] {{.*}} : $@convention(c) (@in_guaranteed Owner) -> @lifetime(borrow 0) @autoreleased View
 // CHECK: sil [clang getViewFromFirst] {{.*}} : $@convention(c) (@in_guaranteed Owner, @in_guaranteed Owner) -> @lifetime(borrow 0) @autoreleased View
@@ -86,6 +89,7 @@ private:
 // CHECK: sil [clang getViewFromEither] {{.*}} : $@convention(c) (@guaranteed View, @guaranteed View) -> @lifetime(copy 0, copy 1) @autoreleased View
 // CHECK: sil [clang View.init] {{.*}} : $@convention(c) () -> @lifetime(immortal) @out View
 // CHECK: sil [clang OtherView.init] {{.*}} : $@convention(c) (@guaranteed View) -> @lifetime(copy 0) @out OtherView
+// CHECK: sil [clang returnsImmortal] {{.*}} : $@convention(c) () -> @lifetime(immortal) @autoreleased View
 
 //--- test.swift
 
@@ -102,4 +106,5 @@ public func test() {
     let _ = getViewFromEither(v1, v2)
     let defaultView = View()
     let _ = OtherView(defaultView)
+    let _ = returnsImmortal()
 }