[Clang importer] Allow noncopyable C structs to define "destroy" operation #83364

DougGregor · 2025-07-28T07:07:21Z

A C struct can be imported as noncopyable, but C doesn't have
destructors, so there is no way to provide user-defined logic to
perform the destruction. Introduce a new swift_attr that applies to
imported noncopyable types and which provides such a "destroy"
operation. It can be used like this:

typedef struct __attribute__((swift_attr("~Copyable")))
               __attribute__((swift_attr("destroy:wgpuAdapterInfoFreeMembers")))
  WGPUAdapterInfo { /*...*/ } WGPUAdapterInfo;

void wgpuAdapterInfoFreeMembers(WGPUAdapterInfo adapterInfo);

This will bring the WGPUAdapterInfo struct in as a noncopyable type
that will be cleaned up by calling wgpuAdapterInfoFreeMembers once it
is no longer in use.

Implements rdar://156889370.

In C interoperability mode, respect the ~Copyable annotation on C structs to import them as ~Copyable types. Fixes rdar://156877772.

…ation A C struct can be imported as noncopyable, but C doesn't have destructors, so there is no way to provide user-defined logic to perform the destruction. Introduce a new swift_attr that applies to imported noncopyable types and which provides such a "destroy" operation. It can be used like this: typedef struct __attribute__((swift_attr("~Copyable"))) __attribute__((swift_attr("destroy:wgpuAdapterInfoFreeMembers"))) WGPUAdapterInfo { /*...*/ } WGPUAdapterInfo; void wgpuAdapterInfoFreeMembers(WGPUAdapterInfo adapterInfo); This will bring the WGPUAdapterInfo struct in as a noncopyable type that will be cleaned up by calling wgpuAdapterInfoFreeMembers once it is no longer in use. Implements rdar://156889370.

DougGregor · 2025-07-28T07:07:30Z

@swift-ci please smoke test

Xazax-hun

This is great! Some nits inline.

Xazax-hun · 2025-07-28T11:47:34Z

lib/ClangImporter/ImportDecl.cpp

+        if (!attributeName.starts_with("destroy:"))
+          continue;
+
+        auto destroyFuncName = attributeName.drop_front(strlen("destroy:"));


Suggested change

if (!attributeName.starts_with("destroy:"))

continue;

auto destroyFuncName = attributeName.drop_front(strlen("destroy:"));

if (!attributeName.consume_front("destroy:"))

continue;

I didn't know about consume_front, thanks!

Xazax-hun · 2025-07-28T11:51:04Z

lib/ClangImporter/ImportDecl.cpp

+    /// Function body synthesizer for a deinit of a noncopyable type, which
+    /// passes "self" to the given "destroy" function.
+    static std::pair<BraceStmt *, bool>
+    synthesizeDeinitBodyForCustomDestroy(


I wonder if this (and co) should go to SwiftDeclSynthesizer.cpp.

Xazax-hun · 2025-07-28T11:53:03Z

test/Interop/C/struct/Inputs/module.modulemap

+
+module NoncopyableStructs {
+  header "noncopyable-struct.h"
+}


Nit: missing new line.

Xazax-hun · 2025-07-28T11:54:48Z

test/Interop/C/struct/Inputs/noncopyable-struct.h

+  float x, y;
+} NonCopyable;
+
+typedef struct __attribute__((swift_attr("~Copyable"))) __attribute__((swift_attr("destroy:freeNonCopyableWithDeinit"))) NonCopyableWithDeinit {


Do we plan to support destroy: with copyable types? If not, should we diagnose when people try to add it to a copyable type?

Maybe we should have a diagnostic when people try to add it to a C++ type that already has a non-trivial dtor.

Copyable value types in Swift don't support user-defined deinit, so we shouldn't try to support it for C structs, either.

A diagnostic when there's a destroy: on a copyable type does make sense, though.

I don't know what to do about C++ types a non-trivial destructor. Warn about and ignore the destroy: operation, perhaps?

Warn about and ignore the destroy: operation, perhaps?

I think that makes sense.

I've done that in the latest commit

test/Interop/C/struct/Inputs/noncopyable-struct.h

When we cannot respect the "destroy:" annotation, mark the type as deprecated with a message thst says why there is a problem. There are various potential problems: * Multiple conflicting destroy functions * Destroy functions that don't meet the pattern * Type isn't imported as a move-only type * Type has a non-trivial destructor (in C++)

DougGregor · 2025-07-28T20:56:37Z

@swift-ci please smoke test

This packages up the ~Copyable and "destroy" attributes in a macro.

DougGregor · 2025-07-29T02:10:53Z

@swift-ci please smoke test

DougGregor · 2025-07-29T06:39:20Z

@swift-ci please smoke test windows

Xazax-hun

Thank you, this looks awesome! I did some nitpicking inline.

Xazax-hun · 2025-07-29T09:05:10Z

lib/ClangImporter/SwiftBridging/swift/bridging

@@ -45,7 +45,7 @@
 #define _CXX_INTEROP_CONCAT(...) \
  _CXX_INTEROP_CONCAT_(__VA_ARGS__,,,,,,,,,,,,,,,,,)

-/// Specifies that a C++ `class` or `struct` is reference-counted using
+/// Specifies that a C `class` or `struct` is reference-counted using


Suggested change

/// Specifies that a C `class` or `struct` is reference-counted using

/// Specifies that a `class` or `struct` is reference-counted using

Xazax-hun · 2025-07-29T09:05:51Z

lib/ClangImporter/SwiftBridging/swift/bridging

 #define SWIFT_NONCOPYABLE \
  __attribute__((swift_attr("~Copyable")))

+/// Specifies that a class or struct should be imported as a non-copyable


Suggested change

/// Specifies that a class or struct should be imported as a non-copyable

/// Specifies that a `class` or `struct` should be imported as a non-copyable

Xazax-hun · 2025-07-29T09:06:13Z

lib/ClangImporter/SwiftBridging/swift/bridging

 #define SWIFT_NONCOPYABLE \
  __attribute__((swift_attr("~Copyable")))

+/// Specifies that a class or struct should be imported as a non-copyable
+/// Swift value type that calls the given _destroy function when a value is no


Suggested change

/// Swift value type that calls the given _destroy function when a value is no

/// Swift value type that calls the given `_destroy` function when a value is no

Xazax-hun · 2025-07-29T09:07:09Z

lib/ClangImporter/SwiftBridging/swift/bridging

+///   // once mt2 is unused, Swift will call mytypeFreeMembers(mt2)
+#define SWIFT_NONCOPYABLE_WITH_DESTROY(_destroy) \


Suggested change

/// // once mt2 is unused, Swift will call mytypeFreeMembers(mt2)

#define SWIFT_NONCOPYABLE_WITH_DESTROY(_destroy) \

/// // once mt2 is unused, Swift will call mytypeFreeMembers(mt2)

/// ```

#define SWIFT_NONCOPYABLE_WITH_DESTROY(_destroy) \

lib/ClangImporter/SwiftDeclSynthesizer.cpp

DougGregor · 2025-07-29T15:27:14Z

@swift-ci please smoke test

DougGregor · 2025-07-29T15:28:48Z

I also did a pass through <swift/bridging> to remove unnecessary mentions of C++ throughout. Only a few things there are actually C++-specific.

DougGregor · 2025-07-29T17:43:44Z

@swift-ci please smoke test macOS

DougGregor added 2 commits July 27, 2025 20:06

[Clang importer] Allow C structs to be noncopyable, too

21b9b9f

In C interoperability mode, respect the ~Copyable annotation on C structs to import them as ~Copyable types. Fixes rdar://156877772.

DougGregor requested review from zoecarver, egorzhdan, Xazax-hun, j-hui, fahadnayyar, susmonteiro, hnrklssn, jckarter, rjmccall, beccadax and ian-twilightcoder as code owners July 28, 2025 07:07

Xazax-hun reviewed Jul 28, 2025

View reviewed changes

DougGregor added 2 commits July 28, 2025 08:30

Fix test for Windows and add missing newline to header

0821009

DougGregor enabled auto-merge July 28, 2025 20:56

DougGregor added 3 commits July 28, 2025 15:51

Add SWIFT_NONCOPYABLE_WITH_DESTROY to swift/bridging header

8909f24

This packages up the ~Copyable and "destroy" attributes in a macro.

Add missing test for newtype'd struct imported as an option set

8680de4

Disable part of this test on Windows

34545e0

Xazax-hun approved these changes Jul 29, 2025

View reviewed changes

DougGregor added 2 commits July 29, 2025 08:05

Split out the part of the test that's failing on Windows

71b96e9

Documentation comment cleanups

2e7e884

Xazax-hun approved these changes Jul 29, 2025

View reviewed changes

DougGregor merged commit 9194fa0 into swiftlang:main Jul 30, 2025
3 checks passed

DougGregor deleted the noncopyable-imported-c-struct-destroy branch July 30, 2025 04:02

	/// Specifies that a C `class` or `struct` is reference-counted using
	/// Specifies that a `class` or `struct` is reference-counted using

	/// Specifies that a class or struct should be imported as a non-copyable
	/// Specifies that a `class` or `struct` should be imported as a non-copyable

	/// Swift value type that calls the given _destroy function when a value is no
	/// Swift value type that calls the given `_destroy` function when a value is no

		/// // once mt2 is unused, Swift will call mytypeFreeMembers(mt2)
		#define SWIFT_NONCOPYABLE_WITH_DESTROY(_destroy) \

[Clang importer] Allow noncopyable C structs to define "destroy" operation #83364

[Clang importer] Allow noncopyable C structs to define "destroy" operation #83364

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DougGregor commented Jul 28, 2025

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DougGregor commented Jul 28, 2025

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DougGregor commented Jul 29, 2025

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DougGregor Jul 28, 2025 •

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