[cxx-interop] Introduce type-level annotations to specify default ownership convention for C++ foreign reference return values (swiftlang#81093)

In Swift 6.1, we introduced `SWIFT_RETURNS_RETAINED` and
`SWIFT_RETURNS_UNRETAINED` annotations for C++ APIs to explicitly
specify the ownership convention of `SWIFT_SHARED_REFERENCE` type return
values.

Currently the Swift compiler emits warnings for unannotated C++ APIs
returning `SWIFT_SHARED_REFERENCE` types. We've received some feedback
that people are finding these warnings useful to get a reminder to
annotate their APIs. While this improves correctness , it also imposes a
high annotation burden on adopters — especially in large C++ codebases.

This patch addresses that burden by introducing two new type-level
annotations:
- `SWIFT_RETURNED_AS_RETAINED_BY_DEFAULT`
- `SWIFT_RETURNED_AS_UNRETAINED_BY_DEFAULT`

These annotations allow developers to specify a default ownership
convention for all C++ APIs returning a given
`SWIFT_SHARED_REFERENCE`-annotated type, unless explicitly overridden at
the API by using `SWIFT_RETURNS_RETAINED` or `SWIFT_RETURNS_UNRETAINED`.
If a C++ class inherits from a base class annotated with
`SWIFT_RETURNED_AS_RETAINED_BY_DEFAULT` or
`SWIFT_RETURNED_AS_UNRETAINED_BY_DEFAULT`, the derived class
automatically inherits the default ownership convention unless it is
explicitly overridden. This strikes a balance between safety/correctness
and usability:

- It avoids the need to annotate every API individually.
- It retains the ability to opt out of the default at the API level when
needed.
- To verify correctness, the user can just remove the
`SWIFT_RETURNED_AS_(UN)RETAINED_BY_DEFAULT` annotation from that type
and they will start seeing the warnings on all the unannotated C++ APIs
returning that `SWIFT_SHARED_REFERENCE` type. They can add
`SWIFT_RETURNS_(UN)RETAINED` annotation at each API in which they want a
different behaviour than the default. Then they can reintroduce the
`SWIFT_RETURNED_AS_(UN)RETAINED_BY_DEFAULT` at the type level to
suppress the warnings on remaining unannotated APIs.

A global default ownership convention (like always return
`unretained`/`unowned`) was considered but it would weaken the
diagnostic signal and remove valuable guardrails that help detect
use-after-free bugs and memory leaks in absence of
`SWIFT_RETURNS_(UN)RETAINED` annotations. In the absence of these
annotations when Swift emits the unannotated API warning, the current
fallback behavior (e.g. relying on heuristics based on API name such as
`"create"`, `"copy"`, `"get"`) is derived from Objective-C interop but
is ill-suited for C++, which has no consistent naming patterns for
ownership semantics.

Several codebases are expected to have project-specific conventions,
such as defaulting to unretained except for factory methods and
constructors. A type-level default seems like the most precise and
scalable mechanism to support such patterns. It integrates cleanly with
existing `SWIFT_SHARED_REFERENCE` usage and provides a per-type opt-in
mechanism without global silencing of ownership diagnostics.

This addition improves ergonomics while preserving the safety benefits
of explicit annotations and diagnostics.

rdar://145453509

Author: fahadnayyar

include/swift/ClangImporter/ClangImporter.h

@@ -19,6 +19,7 @@
 #include "swift/AST/Attr.h"
 #include "swift/AST/AttrKind.h"
 #include "swift/AST/ClangModuleLoader.h"
+#include "clang/AST/Attr.h"
 #include "clang/Basic/Specifiers.h"
 #include "llvm/Support/VirtualFileSystem.h"
 
@@ -70,6 +71,7 @@ namespace dependencies {
 }
 
 namespace swift {
+enum class ResultConvention : uint8_t;
 class ASTContext;
 class CompilerInvocation;
 class ClangImporterOptions;
@@ -802,6 +804,88 @@ bool isClangNamespace(const DeclContext *dc);
 /// specialized class templates are instead imported as unspecialized.
 bool isSymbolicCircularBase(const clang::CXXRecordDecl *templatedClass,
                             const clang::RecordDecl *base);
+
+/// Match a `[[swift_attr("...")]]` annotation on the given Clang decl.
+///
+/// \param decl The Clang declaration to inspect.
+/// \param patterns List of (attribute name, value) pairs.
+/// \returns The value for the first matching attribute, or `std::nullopt`.
+template <typename T>
+std::optional<T>
+matchSwiftAttr(const clang::Decl *decl,
+               llvm::ArrayRef<std::pair<llvm::StringRef, T>> patterns) {
+  if (!decl || !decl->hasAttrs())
+    return std::nullopt;
+
+  for (const auto *attr : decl->getAttrs()) {
+    if (const auto *swiftAttr = llvm::dyn_cast<clang::SwiftAttrAttr>(attr)) {
+      for (const auto &p : patterns) {
+        if (swiftAttr->getAttribute() == p.first)
+          return p.second;
+      }
+    }
+  }
+  return std::nullopt;
+}
+
+/// Like `matchSwiftAttr`, but also searches C++ base classes.
+///
+/// \param decl The Clang declaration to inspect.
+/// \param patterns List of (attribute name, value) pairs.
+/// \returns The matched value from this decl or its bases, or `std::nullopt`.
+template <typename T>
+std::optional<T> matchSwiftAttrConsideringInheritance(
+    const clang::Decl *decl,
+    llvm::ArrayRef<std::pair<llvm::StringRef, T>> patterns) {
+  if (!decl)
+    return std::nullopt;
+
+  if (auto match = matchSwiftAttr<T>(decl, patterns))
+    return match;
+
+  if (const auto *recordDecl = llvm::dyn_cast<clang::CXXRecordDecl>(decl)) {
+    std::optional<T> result;
+    recordDecl->forallBases([&](const clang::CXXRecordDecl *base) -> bool {
+      if (auto baseMatch = matchSwiftAttr<T>(base, patterns)) {
+        result = baseMatch;
+        return false;
+      }
+
+      return true;
+    });
+
+    return result;
+  }
+
+  return std::nullopt;
+}
+
+/// Matches a `swift_attr("...")` on the record type pointed to by the given
+/// Clang type, searching base classes if it's a C++ class.
+///
+/// \param type A Clang pointer type.
+/// \param patterns List of attribute name-value pairs to match.
+/// \returns Matched value or std::nullopt.
+template <typename T>
+std::optional<T> matchSwiftAttrOnRecordPtr(
+    const clang::QualType &type,
+    llvm::ArrayRef<std::pair<llvm::StringRef, T>> patterns) {
+  if (const auto *ptrType = type->getAs<clang::PointerType>()) {
+    if (const auto *recordDecl = ptrType->getPointeeType()->getAsRecordDecl()) {
+      return matchSwiftAttrConsideringInheritance<T>(recordDecl, patterns);
+    }
+  }
+  return std::nullopt;
+}
+
+/// Determines the C++ reference ownership convention for the return value
+/// using `SWIFT_RETURNS_(UN)RETAINED` on the API; falls back to
+/// `SWIFT_RETURNED_AS_(UN)RETAINED_BY_DEFAULT` on the pointee record type.
+///
+/// \param decl The Clang function or method declaration to inspect.
+/// \returns Matched `ResultConvention`, or `std::nullopt` if none applies.
+std::optional<ResultConvention>
+getCxxRefConventionWithAttrs(const clang::Decl *decl);
 } // namespace importer
 
 struct ClangInvocationFileMapping {

lib/ClangImporter/ClangImporter.cpp

@@ -8794,3 +8794,37 @@ bool importer::isSymbolicCircularBase(const clang::CXXRecordDecl *symbolicClass,
   return classTemplate->getCanonicalDecl() ==
          specializedBase->getSpecializedTemplate()->getCanonicalDecl();
 }
+
+std::optional<ResultConvention>
+swift::importer::getCxxRefConventionWithAttrs(const clang::Decl *decl) {
+  using RC = ResultConvention;
+
+  if (auto result =
+          matchSwiftAttr<RC>(decl, {{"returns_unretained", RC::Unowned},
+                                    {"returns_retained", RC::Owned}}))
+    return result;
+
+  const clang::Type *returnTy = nullptr;
+  if (const auto *func = llvm::dyn_cast<clang::FunctionDecl>(decl))
+    returnTy = func->getReturnType().getTypePtrOrNull();
+  else if (const auto *method = llvm::dyn_cast<clang::ObjCMethodDecl>(decl))
+    returnTy = method->getReturnType().getTypePtrOrNull();
+
+  if (!returnTy)
+    return std::nullopt;
+
+  const clang::Type *desugaredReturnTy =
+      returnTy->getUnqualifiedDesugaredType();
+
+  if (const auto *ptrType =
+          llvm::dyn_cast<clang::PointerType>(desugaredReturnTy)) {
+    if (const clang::RecordDecl *record =
+            ptrType->getPointeeType()->getAsRecordDecl()) {
+      return matchSwiftAttrConsideringInheritance<RC>(
+          record, {{"returned_as_unretained_by_default", RC::Unowned},
+                   {"returned_as_retained_by_default", RC::Owned}});
+    }
+  }
+
+  return std::nullopt;
+}

lib/ClangImporter/ImportDecl.cpp

@@ -3665,6 +3665,12 @@ namespace {
             unannotatedAPIWarningNeeded = true;
           }
 
+          if (importer::matchSwiftAttrOnRecordPtr<bool>(
+                  retType, {{"returned_as_retained_by_default", true},
+                            {"returned_as_unretained_by_default", true}})) {
+            unannotatedAPIWarningNeeded = false;
+          }
+
           if (unannotatedAPIWarningNeeded) {
             HeaderLoc loc(decl->getLocation());
             Impl.diagnose(loc, diag::no_returns_retained_returns_unretained,

lib/ClangImporter/SwiftBridging/swift/bridging

@@ -191,6 +191,40 @@
 #define SWIFT_RETURNS_UNRETAINED                                               \
   __attribute__((swift_attr("returns_unretained")))
 
+/// Applied to a C++ foreign reference type annotated with
+/// SWIFT_SHARED_REFERENCE. Indicates that C++ APIs returning this type are
+/// assumed to return an owned (+1) value by default, unless explicitly annotated with
+/// SWIFT_RETURNS_UNRETAINED.
+///
+/// For example:
+/// ```c++
+/// struct SWIFT_SHARED_REFERENCE(retainFoo, releaseFoo)
+/// SWIFT_RETURNED_AS_RETAINED_BY_DEFAULT
+/// Foo { ... };
+/// ```
+///
+/// In Swift, C++ APIs returning `Foo*` will be assumed to return an owned
+/// value.
+#define SWIFT_RETURNED_AS_RETAINED_BY_DEFAULT                                      \
+  __attribute__((swift_attr("returned_as_retained_by_default")))
+
+/// Applied to a C++ foreign reference type annotated with
+/// SWIFT_SHARED_REFERENCE. Indicates that C++ APIs returning this type are
+/// assumed to return an unowned (+0) value by default, unless explicitly annotated
+/// with SWIFT_RETURNS_RETAINED.
+///
+/// For example:
+/// ```c++
+/// struct SWIFT_SHARED_REFERENCE(retainBar, releaseBar)
+/// SWIFT_RETURNED_AS_UNRETAINED_BY_DEFAULT
+/// Bar { ... };
+/// ```
+///
+/// In Swift, C++ APIs returning `Bar*` will be assumed to return an unowned
+/// value.
+#define SWIFT_RETURNED_AS_UNRETAINED_BY_DEFAULT                                    \
+  __attribute__((swift_attr("returned_as_unretained_by_default")))
+
 /// Specifies that the non-public members of a C++ class, struct, or union can
 /// be accessed from extensions of that type, in the given file ID.
 ///

lib/SIL/IR/SILFunctionType.cpp

@@ -1345,23 +1345,18 @@ class Conventions {
     llvm_unreachable("unhandled ownership");
   }
 
-  // Determines owned/unowned ResultConvention of the returned value based on
-  // returns_retained/returns_unretained attribute.
+  // Determines the ownership ResultConvention (owned/unowned) of the return
+  // value using the SWIFT_RETURNS_(UN)RETAINED annotation on the C++ API; if
+  // not explicitly annotated, falls back to the
+  // SWIFT_RETURNED_AS_(UN)RETAINED_BY_DEFAULT annotation on the C++
+  // SWIFT_SHARED_REFERENCE type.
   std::optional<ResultConvention>
   getCxxRefConventionWithAttrs(const TypeLowering &tl,
                                const clang::Decl *decl) const {
-    if (tl.getLoweredType().isForeignReferenceType() && decl->hasAttrs()) {
-      for (const auto *attr : decl->getAttrs()) {
-        if (const auto *swiftAttr = dyn_cast<clang::SwiftAttrAttr>(attr)) {
-          if (swiftAttr->getAttribute() == "returns_unretained") {
-            return ResultConvention::Unowned;
-          } else if (swiftAttr->getAttribute() == "returns_retained") {
-            return ResultConvention::Owned;
-          }
-        }
-      }
-    }
-    return std::nullopt;
+    if (!tl.getLoweredType().isForeignReferenceType())
+      return std::nullopt;
+
+    return importer::getCxxRefConventionWithAttrs(decl);
   }
 };
 

test/Interop/Cxx/foreign-reference/Inputs/cxx-functions-and-methods-returning-frt.h

@@ -1,4 +1,5 @@
 #pragma once
+#include "visibility.h"
 #include <functional>
 
 // FRT or SWIFT_SHARED_REFERENCE type
@@ -326,3 +327,157 @@ struct Derived : Base {
   FRTStruct *_Nonnull VirtualMethodReturningFRTUnowned() override
       __attribute__((swift_attr("returns_unretained")));
 };
+
+SWIFT_BEGIN_NULLABILITY_ANNOTATIONS
+
+namespace DefaultOwnershipConventionOnCXXForeignRefType {
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:defRetain1")))
+__attribute__((swift_attr("release:defRelease1"))) 
+__attribute__((swift_attr("returned_as_retained_by_default"))) RefTyDefRetained {};
+
+RefTyDefRetained *returnRefTyDefRetained() { return new RefTyDefRetained(); }
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:defRetain2")))
+__attribute__((swift_attr("release:defRelease2"))) 
+__attribute__((swift_attr("returned_as_unretained_by_default"))) RefTyDefUnretained {};
+
+RefTyDefUnretained *returnRefTyDefUnretained() {
+  return new RefTyDefUnretained();
+}
+} // namespace DefaultOwnershipConventionOnCXXForeignRefType
+
+void defRetain1(
+    DefaultOwnershipConventionOnCXXForeignRefType::RefTyDefRetained *v) {};
+void defRelease1(
+    DefaultOwnershipConventionOnCXXForeignRefType::RefTyDefRetained *v) {};
+
+void defRetain2(
+    DefaultOwnershipConventionOnCXXForeignRefType::RefTyDefUnretained *v) {};
+void defRelease2(
+    DefaultOwnershipConventionOnCXXForeignRefType::RefTyDefUnretained *v) {};
+
+namespace FunctionAnnotationHasPrecedence {
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:defaultRetain1")))
+__attribute__((swift_attr("release:defaultRelease1")))
+__attribute__((swift_attr("returned_as_unretained_by_default"))) RefTyDefUnretained {};
+
+RefTyDefUnretained *returnRefTyDefUnretained() {
+  return new RefTyDefUnretained();
+}
+RefTyDefUnretained *returnRefTyDefUnretainedAnnotatedRetained()
+    __attribute__((swift_attr("returns_retained"))) {
+  return new RefTyDefUnretained();
+}
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:defaultRetain2")))
+__attribute__((swift_attr("release:defaultRelease2"))) 
+__attribute__((swift_attr("returned_as_retained_by_default"))) RefTyDefRetained {};
+
+RefTyDefRetained *returnRefTyDefRetained() { return new RefTyDefRetained(); }
+RefTyDefRetained *returnRefTyDefRetainedAnnotatedUnRetained()
+__attribute__((swift_attr("returns_unretained"))) {
+  return new RefTyDefRetained();
+}
+
+} // namespace FunctionAnnotationHasPrecedence
+
+void defaultRetain1(FunctionAnnotationHasPrecedence::RefTyDefUnretained *v) {};
+void defaultRelease1(FunctionAnnotationHasPrecedence::RefTyDefUnretained *v) {};
+
+void defaultRetain2(FunctionAnnotationHasPrecedence::RefTyDefRetained *v) {};
+void defaultRelease2(FunctionAnnotationHasPrecedence::RefTyDefRetained *v) {};
+
+namespace DefaultOwnershipSuppressUnannotatedAPIWarning {
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:rretain")))
+__attribute__((swift_attr("release:rrelease"))) RefType {};
+
+RefType *returnRefType() { return new RefType(); } // expected-warning {{'returnRefType' should be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED as it is returning a SWIFT_SHARED_REFERENCE}}
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:dretain")))
+__attribute__((swift_attr("release:drelease"))) 
+__attribute__((swift_attr("returned_as_retained_by_default"))) RefTyDefRetained {};
+
+RefTyDefRetained *returnRefTyDefRetained() { return new RefTyDefRetained(); }
+
+} // namespace DefaultOwnershipSuppressUnannotatedAPIWarning
+
+void rretain(DefaultOwnershipSuppressUnannotatedAPIWarning::RefType *v) {};
+void rrelease(DefaultOwnershipSuppressUnannotatedAPIWarning::RefType *v) {};
+
+void dretain(
+    DefaultOwnershipSuppressUnannotatedAPIWarning::RefTyDefRetained *v) {};
+void drelease(
+    DefaultOwnershipSuppressUnannotatedAPIWarning::RefTyDefRetained *v) {};
+
+namespace DefaultOwnershipInheritance {
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:baseRetain")))
+__attribute__((swift_attr("release:baseRelease")))
+__attribute__((swift_attr("returned_as_retained_by_default"))) BaseType {};
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:derivedRetain")))
+__attribute__((swift_attr("release:derivedRelease"))) DerivedType
+    : public BaseType {};
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:derivedRetain2")))
+__attribute__((swift_attr("release:derivedRelease2"))) DerivedType2
+    : public DerivedType {};
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:derivedRetain3")))
+__attribute__((swift_attr("release:derivedRelease3"))) 
+__attribute__((swift_attr("returned_as_unretained_by_default"))) DerivedOverride
+    : public DerivedType {};
+
+BaseType *returnBaseType() { return new BaseType(); }
+DerivedType *returnDerivedType() { return new DerivedType(); }
+DerivedType2 *returnDerivedType2() { return new DerivedType2(); }
+DerivedOverride *returnDerivedOverride() { return new DerivedOverride(); }
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:bRetain")))
+__attribute__((swift_attr("release:bRelease"))) BaseTypeNonDefault {};
+
+struct __attribute__((swift_attr("import_reference")))
+__attribute__((swift_attr("retain:dRetain")))
+__attribute__((swift_attr("release:dRelease"))) DerivedTypeNonDefault
+    : public BaseTypeNonDefault {};
+
+BaseTypeNonDefault *returnBaseTypeNonDefault() { // expected-warning {{'returnBaseTypeNonDefault' should be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED as it is returning a SWIFT_SHARED_REFERENCE}}
+  return new BaseTypeNonDefault();
+}
+DerivedTypeNonDefault *returnDerivedTypeNonDefault() { // expected-warning {{'returnDerivedTypeNonDefault' should be annotated with either SWIFT_RETURNS_RETAINED or SWIFT_RETURNS_UNRETAINED as it is returning a SWIFT_SHARED_REFERENCE}}
+  return new DerivedTypeNonDefault();
+}
+
+} // namespace DefaultOwnershipInheritance
+
+void baseRetain(DefaultOwnershipInheritance::BaseType *v) {};
+void baseRelease(DefaultOwnershipInheritance::BaseType *v) {};
+
+void derivedRetain(DefaultOwnershipInheritance::DerivedType *v) {};
+void derivedRelease(DefaultOwnershipInheritance::DerivedType *v) {};
+
+void derivedRetain2(DefaultOwnershipInheritance::DerivedType2 *v) {};
+void derivedRelease2(DefaultOwnershipInheritance::DerivedType2 *v) {};
+
+void derivedRetain3(DefaultOwnershipInheritance::DerivedOverride *v) {};
+void derivedRelease3(DefaultOwnershipInheritance::DerivedOverride *v) {};
+
+void bRetain(DefaultOwnershipInheritance::BaseTypeNonDefault *v) {};
+void bRelease(DefaultOwnershipInheritance::BaseTypeNonDefault *v) {};
+
+void dRetain(DefaultOwnershipInheritance::DerivedTypeNonDefault *v) {};
+void dRelease(DefaultOwnershipInheritance::DerivedTypeNonDefault *v) {};
+
+SWIFT_END_NULLABILITY_ANNOTATIONS