SwiftSyntax crashes macro expansion when the macro depends on package that imports SwiftSyntax

[stansmida]

Description

SwiftSyntax crashes in macro CompilerPlugin when the macro depends on package that imports SwiftSyntax.

SwiftCompile normal arm64 Compiling\ main.swift /Users/stan/Desktop/CrashingSwiftSyntax/CrashingMacro/Sources/CrashingMacroClient/main.swift (in target 'CrashingMacroClient' from project 'CrashingMacro')

Simultaneous accesses to 0x80b000c68, but modification requires exclusive access.
Previous access (a modification) started at CrashingMacroMacros`ParsingRawSyntaxArena.sourceBuffer.modify + 40 (0x103032d04).
Current access (a read) started at:
0    libswiftCore.dylib                 0x00000001977876f4 swift::runtime::AccessSet::insert(swift::runtime::Access*, void*, void*, swift::ExclusivityFlags) + 476
1    libswiftCore.dylib                 0x0000000197787988 swift_beginAccess + 84
2    CrashingMacroMacros                0x00000001030333d8 ParsingRawSyntaxArena.sourceBufferContains(_:) + 92
3    CrashingMacroMacros                0x00000001030332e0 ParsingRawSyntaxArena.contains(text:) + 128
4    CrashingMacroMacros                0x0000000103029348 implicit closure swiftlang/swift-syntax#1 in static RawSyntax.parsedToken(kind:wholeText:textRange:presence:tokenDiagnostic:arena:) + 52
5    CrashingMacroMacros                0x0000000103018c7c assert(_:_:file:line:) + 92
6    CrashingMacroMacros                0x0000000103029110 static RawSyntax.parsedToken(kind:wholeText:textRange:presence:tokenDiagnostic:arena:) + 256
7    CrashingMacroMacros                0x0000000103038304 RawTokenSyntax.init(kind:wholeText:textRange:presence:tokenDiagnostic:arena:) + 120
8    CrashingMacroMacros                0x00000001038b1148 Parser.expect(_:) + 1240
9    CrashingMacroMacros                0x0000000103878a5c Parser.parsePrimaryExpression(pattern:flavor:) + 1452
10   CrashingMacroMacros                0x00000001038762a4 Parser.parseUnaryExpression(flavor:pattern:) + 556
11   CrashingMacroMacros                0x0000000103873b88 Parser.parseSequenceExpressionElement(flavor:pattern:) + 696
12   CrashingMacroMacros                0x0000000103873150 Parser.parseSequenceExpression(flavor:pattern:) + 72
13   CrashingMacroMacros                0x0000000103870774 Parser.parseExpression(flavor:pattern:) + 124
14   CrashingMacroMacros                0x000000010388a8a4 static ExprSyntax.parse(from:) + 36
15   CrashingMacroMacros                0x00000001038122e0 specialized ParsedSyntaxRegistry.parse(source:kind:) + 152
16   CrashingMacroMacros                0x0000000103810f34 SourceManager.add(_:foldingWith:) + 472
17   CrashingMacroMacros                0x000000010380ef7c specialized PluginProviderMessageHandler.expandFreestandingMacro(macro:macroRole:discriminator:expandingSyntax:lexicalContext:) + 200
18   CrashingMacroMacros                0x00000001037f243c PluginProviderMessageHandler.handleMessage(_:) + 604
19   CrashingMacroMacros                0x00000001037f28c0 protocol witness for PluginMessageHandler.handleMessage(_:) in conformance PluginProviderMessageHandler<A> + 20
20   CrashingMacroMacros                0x00000001037f2120 CompilerPluginMessageListener.handleNextMessage() + 204
21   CrashingMacroMacros                0x00000001037f20d0 CompilerPluginMessageListener.main() + 28
22   CrashingMacroMacros                0x00000001037f0b38 static CompilerPlugin.main() + 364
23   CrashingMacroMacros                0x0000000102ffdb84 static CrashingMacroPlugin.$main() + 44
24   CrashingMacroMacros                0x0000000102ffdcf4 main + 28
25   dyld                               0x0000000184284144 start + 7184
Fatal access conflict detected.

Steps to Reproduce

Toolchain: Xcode 26.0.1

1. Create new library and macro package.
2. Have the library depend on SwiftSyntax.
3. Have the macro depend on the library.
4. Build the macro client - crash.

You can try the crashing example at https://github.com/stansmida/crashing-swift-syntax

[stansmida]

This is with the Swift-Syntax Prebuilts enabled.
It seems that problem is caused by two distinct copies of SwiftSyntax loaded into the process.

--disable-experimental-prebuilts does workaround the crash but unfortunately this can't be applied on library level. Any consumer of such macro (that worked prior to the prebuilts) would result in build failure with Failed to receive result from plugin without further information about the problem or the possible workaround.

Slack Thread: https://swift-open-source.slack.com/archives/C04MRUD6VH8/p1758913738586689

[dschaefer2]

Bringing this over to SwiftPM. Libraries that depend on swift-syntax that are then used by macros has always been problematic since we don't know whether the library is also used in a cross compilation. We only support prebuilts for host, this only for macros. Unfortunately the fix will be to automatically disable the prebuilts for that macro, at least until we can express platform dependencies in the build system.

[stansmida]

I’m sorry for filing the issue in the wrong repo — I thought the problem might be related to ParsingRawSyntaxArena.

Anyway, all of my macros have worked perfectly fine when depending on modules that themselves depend on SwiftSyntax. With prebuilts now turned on by default, I consider this a regression. I’m concerned that clients of such macros can’t be given a clear explanation of how to fix their build. I assume there will be more macro authors who have swift-syntax utility libraries for parsing input syntax and producing expansion syntax — surprisingly, though, I haven’t seen anyone else raising this concern here.

Do you think there’s a chance we might see support for expressing platform dependencies in the build system in upcoming versions? Or generally any change that would address this? I’m worried that simply documenting a macro as requiring prebuilts to be disabled will either go unnoticed or discourage adoption. If no proper fix is provided, I fear the inevitable workaround will be to inline these utilities directly into each macro implementation.

[Monsteel]

Any updates on this?
I’ve had to downgrade the SwiftSyntax / SPM package version to 5.10 as newer ones cause crashes during macro expansion.
Hope a proper fix is planned.

[dschaefer2]

Working on it now. You can also disable prebuilts with --disable-experimental-prebuilts as mentioned above if you want to use a newer toolchain.

[Monsteel]

@dschaefer2

Thanks for the quick response and for working on this :)
Using the --disable-experimental-prebuilts flag doesn’t seem ideal from my perspective, since I’m planning to distribute this as a package.

Please let me know if there are any updates on this issue — I’d really appreciate it!

[jeffreyrlewis]

Oh wow .. I thought I was going insane. It never occurred to me that writing a library of SwiftSyntax utilities would be a problem. Thank goodness I found this thread!

[Monsteel]

I’m developing a macro package intended for distribution via Swift Package Manager in an environment using Xcode 26 and Swift 6.2.
I verify the macro expansion results through the test target, following the approach introduced at WWDC.

However, adding the --disable-experimental-prebuilts build option was not suitable for development within Xcode.
Therefore, I’m temporarily using the following workaround during Xcode-based development:

1. Quit Xcode.
2. Issue the command defaults write com.apple.dt.Xcode IDEPackageEnablePrebuilts NO at the command line.
3. Clean out the DerivedData for the project in ~/Library/Developer/Xcode/DerivedData.
4. Restart Xcode and execute the build.

The prebuilt system is a great feature, and I hope it becomes stable and fully usable soon.

Additionally, for package distribution, I plan to include a note recommending that users build with the --disable-experimental-prebuilts option.

[dschaefer2]

The fix I'm working on now unfortunately will just disable the prebuilts in this case. That will be for an upcoming 6.2.x. I'm digging into the build system to see if we can add prebuilts for host libraries in general but that so far that has been a pretty big change that still doesn't work in all scenarios. But we'll want to soak it for a while and make sure people try it out.