[cxx-interop] Support for printing C++ foreign references #81832

susmonteiro · 2025-05-29T11:05:59Z

Adds support for printing a C++ foreign reference in Swift.

Also skips metadata of private fields in C++ records imported as Swift classes, following up on #81035

Resolves rdar://124164228

susmonteiro · 2025-05-29T11:06:10Z

@swift-ci please smoke test

susmonteiro · 2025-06-04T12:09:00Z

@swift-ci please smoke test

susmonteiro · 2025-06-04T16:02:07Z

@swift-ci please smoke test

Xazax-hun

Looks great! Some minor nits inline.

Xazax-hun · 2025-06-04T16:17:34Z

stdlib/public/core/OutputStream.swift

@@ -381,6 +381,10 @@ internal func _adHocPrint_unlocked<T, TargetStream: TextOutputStream>(
            target.write(")")
          }
        }
+      case .foreign:
+        printTypeName(mirror.subjectType)
+        // FRT has no children


Do we want to print the (public) fields similar to structs in the future? I think it is fine to not do it in this PR but we might want to have a rdar for it if this is planned and possible to achieve?

At the moment, we don't emit field metadata for FRT, so we don't have any field information at runtime:

swift/lib/IRGen/GenMeta.cpp

Lines 2006 to 2009 in 68fb206

if (getType()->isForeign())

return;

MetadataLayout = &IGM.getClassMetadataLayout(Type);

swift/lib/IRGen/GenMeta.cpp

Lines 2120 to 2125 in 68fb206

Size getFieldVectorOffset() {

if (!MetadataLayout) return Size(0);

return (MetadataLayout->hasResilientSuperclass()

? MetadataLayout->getRelativeFieldOffsetVectorOffset()

: MetadataLayout->getStaticFieldOffsetVectorOffset());

}

I'm not sure if this is on purpose or it's simply not yet implemented, but I agree it would be useful to figure this out and, potentially, print more information in the future

Xazax-hun · 2025-06-04T16:18:25Z

stdlib/public/core/DebuggerSupport.swift

@@ -147,7 +147,7 @@ public enum _DebuggerSupport {
      return value.map(String.init(reflecting:))
    case .collection?, .dictionary?, .set?, .tuple?:
      return count == 1 ? "1 element" : "\(count) elements"
-    case .`struct`?, .`enum`?, nil:
+    case .`struct`?, .`enum`?, .foreign?, nil:


I wonder if foreign reference types should behave more like classes and print the memory address.

asObjectAddress(...) of a FRT returns 0, so for now I think we shouldn't print the memory address. Most likely this happens for the same reason that we don't have metadata about the fields of FRTs at runtime.

If in the future we decide to work on emitting more metadata for FRTs, it could make sense to add .foreign to the same case as classes. For now, I would keep it as is, if you agree?

If supporting this requires additional work I am fine to leave this as is.

Agreed that we should ideally print an address, also agreed that it doesn't have to be in this PR 😉

test/abi/macOS/x86_64/stdlib.swift

susmonteiro · 2025-06-05T13:52:04Z

@swift-ci please smoke test

susmonteiro · 2025-06-09T09:44:56Z

@swift-ci please test Windows platform

egorzhdan

Thanks! Just a couple of minor comments, and then this should be good to go.

egorzhdan · 2025-06-09T11:50:38Z

stdlib/public/core/DebuggerSupport.swift

@@ -147,7 +147,7 @@ public enum _DebuggerSupport {
      return value.map(String.init(reflecting:))
    case .collection?, .dictionary?, .set?, .tuple?:
      return count == 1 ? "1 element" : "\(count) elements"
-    case .`struct`?, .`enum`?, nil:
+    case .`struct`?, .`enum`?, .foreign?, nil:


Agreed that we should ideally print an address, also agreed that it doesn't have to be in this PR 😉

egorzhdan · 2025-06-09T11:52:26Z

stdlib/public/core/Mirror.swift

@@ -313,6 +313,7 @@ extension Mirror {
  public enum DisplayStyle: Sendable {
    case `struct`, `class`, `enum`, tuple, optional, collection
    case dictionary, `set`
+    @available(SwiftStdlib 6.2, *) case foreign


I think we should call this foreignReference to make it clear that this is about foreign reference types specifically. IRGen uses the term "foreign" to refer to C structs that are imported as value types, and here this means something different.

I see, thanks!

egorzhdan · 2025-06-09T11:52:32Z

stdlib/public/core/Mirror.swift

@@ -481,6 +482,7 @@ public struct Mirror {
  public enum DisplayStyle: Sendable {
    case `struct`, `class`, `enum`, tuple, optional, collection
    case dictionary, `set`
+    @available(SwiftStdlib 6.2, *) case foreign


test/Interop/Cxx/metadata/mirror.swift

susmonteiro · 2025-06-09T12:45:57Z

Agreed that we should ideally print an address, also agreed that it doesn't have to be in this PR 😉

As this isn't a trivial change, I think we could do it in another PR 🙂

susmonteiro · 2025-06-09T15:11:17Z

@swift-ci please smoke test

susmonteiro · 2025-06-10T11:18:15Z

@swift-ci please smoke test

susmonteiro added the c++ interop Feature: Interoperability with C++ label May 29, 2025

susmonteiro force-pushed the susmonteiro/class-metadata-private-fields branch from 4aadc58 to ebdea6a Compare June 4, 2025 12:08

susmonteiro force-pushed the susmonteiro/class-metadata-private-fields branch from ebdea6a to 68fb206 Compare June 4, 2025 15:54

susmonteiro changed the title ~~[cxx-interop] Skip metadata of C++ private fields in ClassContextDescriptorBuilder~~ [cxx-interop] Support from printing C++ foreign references Jun 4, 2025

susmonteiro marked this pull request as ready for review June 4, 2025 16:02

susmonteiro requested review from a team, zoecarver, egorzhdan, Xazax-hun, j-hui, fahadnayyar, hnrklssn and rjmccall as code owners June 4, 2025 16:02

Xazax-hun reviewed Jun 4, 2025

View reviewed changes

susmonteiro changed the title ~~[cxx-interop] Support from printing C++ foreign references~~ [cxx-interop] Support for printing C++ foreign references Jun 5, 2025

susmonteiro force-pushed the susmonteiro/class-metadata-private-fields branch from 68fb206 to 3ec1479 Compare June 5, 2025 13:51

Xazax-hun approved these changes Jun 5, 2025

View reviewed changes

egorzhdan reviewed Jun 9, 2025

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susmonteiro force-pushed the susmonteiro/class-metadata-private-fields branch from 3ec1479 to 9975ac8 Compare June 9, 2025 15:10

[cxx-interop] Support for printing C++ foreign references

848fad0

susmonteiro force-pushed the susmonteiro/class-metadata-private-fields branch from 9975ac8 to 848fad0 Compare June 10, 2025 11:15

susmonteiro merged commit 6164af2 into main Jun 11, 2025
3 checks passed

susmonteiro deleted the susmonteiro/class-metadata-private-fields branch June 11, 2025 10:58

susmonteiro mentioned this pull request Jun 11, 2025

🍒 [cxx-interop] Support for printing C++ foreign references #82185

Open

	if (getType()->isForeign())
	return;

	MetadataLayout = &IGM.getClassMetadataLayout(Type);

	Size getFieldVectorOffset() {
	if (!MetadataLayout) return Size(0);
	return (MetadataLayout->hasResilientSuperclass()
	? MetadataLayout->getRelativeFieldOffsetVectorOffset()
	: MetadataLayout->getStaticFieldOffsetVectorOffset());
	}

[cxx-interop] Support for printing C++ foreign references #81832

[cxx-interop] Support for printing C++ foreign references #81832

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