SILGen: Don't reference external property descriptors for @backDeployed properties.

The property descriptors of `@backDeployed` properties aren't available on OSes
prior to the "back deployed before" OS version, so the descriptors cannot be
referenced by clients that may run against older versions of the library
that defines the property.

See #59214 for prior art.

Resolves rdar://106517386

Author: tshortli

lib/SILGen/SILGen.h

@@ -351,6 +351,11 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
   /// Emits a thunk from an actor function to a potentially distributed call.
   void emitDistributedThunk(SILDeclRef thunk);
 
+  /// Returns true if the given declaration must be referenced through a
+  /// back deployment thunk in a context with the given resilience expansion.
+  bool requiresBackDeploymentThunk(ValueDecl *decl,
+                                   ResilienceExpansion expansion);
+
   /// Emits a thunk that calls either the original function if it is available
   /// or otherwise calls a fallback variant of the function that was emitted
   /// into the client module.

lib/SILGen/SILGenApply.cpp

@@ -280,31 +280,6 @@ static void convertOwnershipConventionsGivenParamInfos(
                   });
 }
 
-static bool shouldApplyBackDeploymentThunk(ValueDecl *decl, ASTContext &ctx,
-                                           ResilienceExpansion expansion) {
-  auto backDeployBeforeVersion = decl->getBackDeployedBeforeOSVersion(ctx);
-  if (!backDeployBeforeVersion)
-    return false;
-
-  // If the context of the application is inlinable then we must always call the
-  // back deployment thunk since we can't predict the deployment targets of
-  // other modules.
-  if (expansion != ResilienceExpansion::Maximal)
-    return true;
-
-  // In resilient function bodies skip calling the back deployment thunk when
-  // the deployment target is high enough that the ABI implementation of the
-  // back deployed function is guaranteed to be available.
-  auto deploymentAvailability = AvailabilityContext::forDeploymentTarget(ctx);
-  auto declAvailability =
-      AvailabilityContext(VersionRange::allGTE(*backDeployBeforeVersion));
-
-  if (deploymentAvailability.isContainedIn(declAvailability))
-    return false;
-
-  return true;
-}
-
 //===----------------------------------------------------------------------===//
 //                                   Callee
 //===----------------------------------------------------------------------===//
@@ -1155,7 +1130,6 @@ class SILGenApply : public Lowering::ExprVisitor<SILGenApply> {
 
   SILDeclRef getDeclRefForStaticDispatchApply(DeclRefExpr *e) {
     auto *afd = cast<AbstractFunctionDecl>(e->getDecl());
-    auto &ctx = SGF.getASTContext();
 
     // A call to a `distributed` function may need to go through a thunk.
     if (callSite && callSite->shouldApplyDistributedThunk()) {
@@ -1164,8 +1138,9 @@ class SILGenApply : public Lowering::ExprVisitor<SILGenApply> {
     }
 
     // A call to `@backDeployed` function may need to go through a thunk.
-    if (shouldApplyBackDeploymentThunk(afd, ctx,
-                                       SGF.F.getResilienceExpansion())) {
+
+    if (SGF.SGM.requiresBackDeploymentThunk(afd,
+                                            SGF.F.getResilienceExpansion())) {
       return SILDeclRef(afd).asBackDeploymentKind(
           SILDeclRef::BackDeploymentKind::Thunk);
     }
@@ -6711,7 +6686,7 @@ SILDeclRef SILGenModule::getAccessorDeclRef(AccessorDecl *accessor,
                                             ResilienceExpansion expansion) {
   auto declRef = SILDeclRef(accessor, SILDeclRef::Kind::Func);
 
-  if (shouldApplyBackDeploymentThunk(accessor, getASTContext(), expansion))
+  if (requiresBackDeploymentThunk(accessor, expansion))
     return declRef.asBackDeploymentKind(SILDeclRef::BackDeploymentKind::Thunk);
 
   return declRef.asForeign(requiresForeignEntryPoint(accessor));

lib/SILGen/SILGenBackDeploy.cpp

@@ -168,6 +168,36 @@ static void emitBackDeployForwardApplyAndReturnOrThrow(
   SGF.B.createBranch(loc, SGF.ReturnDest.getBlock(), directResults);
 }
 
+bool SILGenModule::requiresBackDeploymentThunk(ValueDecl *decl,
+                                               ResilienceExpansion expansion) {
+  auto &ctx = getASTContext();
+  auto backDeployBeforeVersion = decl->getBackDeployedBeforeOSVersion(ctx);
+  if (!backDeployBeforeVersion)
+    return false;
+
+  switch (expansion) {
+  case ResilienceExpansion::Minimal:
+    // In a minimal resilience expansion we must always call the back deployment
+    // thunk since we can't predict the deployment targets of the modules that
+    // might inline the call.
+    return true;
+  case ResilienceExpansion::Maximal:
+    // FIXME: We can skip thunking if we're in the same module.
+    break;
+  }
+
+  // Use of a back deployment thunk is unnecessary if the deployment target is
+  // high enough that the ABI implementation of the back deployed declaration is
+  // guaranteed to be available.
+  auto deploymentAvailability = AvailabilityContext::forDeploymentTarget(ctx);
+  auto declAvailability =
+      AvailabilityContext(VersionRange::allGTE(*backDeployBeforeVersion));
+  if (deploymentAvailability.isContainedIn(declAvailability))
+    return false;
+
+  return true;
+}
+
 void SILGenFunction::emitBackDeploymentThunk(SILDeclRef thunk) {
   // Generate code equivalent to:
   //

lib/SILGen/SILGenExpr.cpp

@@ -3803,6 +3803,12 @@ SILGenModule::emitKeyPathComponentForDecl(SILLocation loc,
       return false;
     }
 
+    // Back deployed properties have the same restrictions as
+    // always-emit-into-client properties.
+    if (requiresBackDeploymentThunk(baseDecl, expansion)) {
+      return false;
+    }
+
     // Properties that only dispatch via ObjC lookup do not have nor
     // need property descriptors, since the selector identifies the
     // storage.

test/SILGen/back_deployed_attr_accessor.swift

@@ -45,4 +45,11 @@ func caller(_ s: TopLevelStruct) {
   // -- Verify the thunk is called
   // CHECK: {{%.*}} = function_ref @$s11back_deploy14TopLevelStructV8propertyACvgTwb : $@convention(method) (TopLevelStruct) -> TopLevelStruct
   _ = s.property
+
+  // -- Verify key path
+  // CHECK: {{%.*}} = keypath $KeyPath<TopLevelStruct, TopLevelStruct>, (root $TopLevelStruct; gettable_property $TopLevelStruct,  id @$s11back_deploy14TopLevelStructV8propertyACvg : $@convention(method) (TopLevelStruct) -> TopLevelStruct, getter @$s11back_deploy14TopLevelStructV8propertyACvpACTK : $@convention(thin) (@in_guaranteed TopLevelStruct) -> @out TopLevelStruct)
+  _ = \TopLevelStruct.property
 }
+
+// CHECK-LABEL: sil shared [thunk] [ossa] @$s11back_deploy14TopLevelStructV8propertyACvpACTK : $@convention(thin) (@in_guaranteed TopLevelStruct) -> @out TopLevelStruct
+// CHECK: function_ref @$s11back_deploy14TopLevelStructV8propertyACvgTwb

test/attr/attr_backDeployed_evolution.swift

@@ -149,6 +149,7 @@ do {
 
   let empty = IntArray.empty
   precondition(empty.values == [])
+  precondition(empty[keyPath: \.values] == [])
 
   var array = IntArray([5])
 
@@ -177,6 +178,7 @@ do {
 do {
   let defaulted = ReferenceIntArray()
   precondition(defaulted.values == [])
+  precondition(defaulted[keyPath: \.values] == [])
 
   let empty = ReferenceIntArray.empty
   precondition(empty.values == [])