Swift 6.0 development snapshots fail to build with Xcode 16 Beta 5 when explicitly built modules are enabled

[mavitalemeta]

Description

• Building with the latest development snapshot and the latest Xcode 16 Beta 5 with Explicitly Built Modules enabled results in build failures in the SDK header include file around unknown type name rsize_t.
• This same error does not occur with Xcode 16 Beta 4.
• This same error does not occur when Explicitly Built Modules is disabled (although we see another issue that is separately tracked by Every package with Xcode + snapshot is crashing the compiler in the same way #75772)
• This error does not occur when using the version of the Swift compiler bundled with Xcode 16 Beta 5.

Build failure snippet from Xcode:

2 errors generated.
/Applications/Xcode_16.0.0_16A5221g_fb.app/Contents/Developer/Platforms/iPhoneSimulator.platform/Developer/SDKs/iPhoneSimulator.sdk/usr/include/_string.h:145:54: error: unknown type name 'rsize_t'; did you mean 'size_t'?
143 | 
144 | __BEGIN_DECLS
145 | errno_t	memset_s(void *__s, rsize_t __smax, int __c, rsize_t __n) __OSX_AVAILABLE_STARTING(__MAC_10_9, __IPHONE_7_0);
    |                                                      `- error: unknown type name 'rsize_t'; did you mean 'size_t'?
146 | __END_DECLS
147 | #endif

/Library/Developer/Toolchains/swift-6.0-DEVELOPMENT-SNAPSHOT-2024-08-08-a.xctoolchain/usr/lib/swift/clang/include/__stddef_size_t.h:18:23: note: 'size_t' declared here
16 | #define _SIZE_T
17 | 
18 | typedef __SIZE_TYPE__ size_t;
   |                       `- note: 'size_t' declared here
19 | 
20 | #endif

Full build errror: Build ExplicitModulesTestApp_2024-08-09T11-50-02.txt

Reproduction

1. Install Xcode 16 Beta 5
2. Install the latest development snapshot package for macOS
   1. As of this writing, this is the “Swift 6.0 Development Snapshot 2024-08-08 (a)” toolchain from https://www.swift.org/install/macos/
3. Choose the development snapshot toolchain in Xcode
   1. “Xcode -> Toolchains -> Swift 6.0 Development Snapshot 2024-08-08 (a)”
4. Open the attached Xcode project at ExplicitlyBuiltModulesProject.zip
   1. Alternatively repro with a default template project: In Xcode 16 Beta 5 create a new default iOS Application project
      1. go to “File -> New -> Project…”
      2. Select the iOS Platform and the “App” template
      3. Interface: Storyboard
      4. Language: Swift
      5. Testing System: None
      6. Storage: None
      7. Enable Explicitly Built Modules (_EXPERIMENTAL_SWIFT_EXPLICIT_MODULES) build setting for the project
         1. Enable it in the project settings, in the Swift Compiler - General settings (search for Explicitly Built Modules)
5. Build the app with Command-B or “Product -> Build”
6. Observe the build output

Expected behavior

• Expected the build to succeed with the latest open source compiler development snapshot, the latest Xcode beta, and Explicitly Built Modules enabled.

Environment

• macOS Sonoma 14.5 (23F79)
• Xcode Version 16.0 beta 5 (16A5221g)
• Swift 6.0 Development Snapshot 2024-08-08 (a)

Additional information

• Testing with Explicitly Built Modules (_EXPERIMENTAL_SWIFT_EXPLICIT_MODULES) set to NO results in an assertion failure tracked by Every package with Xcode + snapshot is crashing the compiler in the same way #75772, but does not show the size_t/rsize_t build issue.
• There are 6 files that were changed in the iPhoneSimulator 18.0 sdk in Xcode 16 beta 5, specifically around how certain types including rsize_t are defined. This newly introduced USE_CLANG_STDDEF is likely related to the issue, but it’s not clear how the bundled version of the swift compiler handles this while the open source development snapshots don’t handle it.
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_ptrdiff_t.h
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_rsize_t.h
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_wchar_t.h
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_null.h
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_size_t.h
  • iPhoneSimulator18.0.sdk/usr/include/sys/_types/_offsetof.h

[NuriAmari]

Should be fixed by llvm/llvm-project#102239