assertMacroExpansion always passes empty array as `conformingTo` parameter of extension macros

[stephencelis]

Description

Previously a bug with conformance macros:

• https://github.com/apple/swift-syntax/issues/1742
• https://github.com/apple/swift-syntax/pull/1773
• https://github.com/apple/swift-syntax/issues/1801

This issue has regressed with the change to extension macros.

Steps to Reproduce

1. Write a macro expansion test for an extension macro.
2. Note that the expanded source does not include the extension.

[ahoppen]

Tracked in Apple’s issue tracker as rdar://113582092

[ahoppen]

@stephencelis Do you have an example where the extension is not included in the expanded source? Because the SendableExtensionMacro in swift-syntax’s test suite works just fine.

https://github.com/apple/swift-syntax/blob/5412040ebb79c529c16a8a8a6bbfe59baf06c5f6/Tests/SwiftSyntaxMacroExpansionTest/MacroSystemTests.swift#L1504-L1522

[stephencelis]

@ahoppen This might be a documentation problem, but the first example of an extension macro I found was this one in the Swift repo, which starts:

https://github.com/apple/swift/blob/7699834154417cb6eea9fc6438cf5a4c3e589dee/lib/Macros/Sources/SwiftMacros/OptionSetMacro.swift#L135-L137

These three lines of code make the macro expansion testing tool never add the conformance, presumably because macro expansion testing is totally decoupled from the library's macro declaration that specifies the allowed conformances.

[ahoppen]

Ah, yes. That’s a current limitation of assertMacroExpansion, which, at the moment, always passes an empty list for conformingTo. We are aware of the issue and are looking into it.

[soumyamahunt]

@ahoppen if there is limitation in inferring conformance for a type, I would expect all the protocols that macro declares conformance for all provided. That way existing tests will not break when migrating to extension macro from member macro.

[ahoppen]

The problem is that in assertMacroExpansion, we currently don’t know which conformances the macro declaration (ie the thing after @attached) contains because assertMacroExpansion only has a mapping from the macro name to the type. We would need to design some way of passing that information in to assertMacroExpansion here.

[DougGregor]

The simplest approach is to pass this information as an extra parameter through assertMacroExpansion, e.g.,

assertMacroExpansion( 
       """ 
       @AddSendableExtension 
       struct MyType { 
       } 
       """, 
       expandedSource: """ 

         struct MyType { 
         } 

         extension MyType: Sendable { 
         } 
         """, 
       macros: testMacros, 
       conformsTo: ["AddSendableExtension": ["Sendable"]],
       indentationWidth: indentationWidth 
     ) 

where the conformsTo parameter has a type like [String: [TypeSyntax]] so we can pass different sets of conformances to each macro.

There are more advanced approaches I could imagine that, for example, depend on passing the actual macro declarations into assertMacroExpansion, e.g., we could pass in this text

@attached(extension, conformances: Sendable)
macro AddSendableExtension() = ...

and then use the conformances in the attached macro role declaration to populate the conformances passed to the macro implementations. It's more automatic, but it's a lot more plumbing.

[soumyamahunt]

@ahoppen anything finalized on the approach yet, I am thinking of creating PR with @DougGregor's suggestion.