MacroExpansionContext.lexicalContext always empty for freestanding macros?

[Jeehut]

Description

I am currently writing a freestanding macro using version 600.0.1 which needs to read some information about its enclosing code. According to this thread on the forums @DougGregor has implemented exactly what I need in #1554 inside the lexicalContext property. But when I call context.lexicalContext I always get an empty array, even though my freestanding macro is placed inside a SwiftUI body within a Button initializer like this:

import SwiftUI
      
struct MyView: View {
   var body: some View {
      Button(#tk("Save Changes")) { 
         self.handleSave()
      }
   }
}

The purpose of my macro is to auto-generate a semantic localization key based on the context. The expected expansion:

import SwiftUI

struct MyView: View {
   var body: some View {
      Button(String(localized: "MyView.Body.Button.saveChanges", defaultValue: "Save Changes")) { 
         self.handleSave()
      }
   }
}

I got everything working, except that I can't access the context. I would expect lexicalContext to be something like ["Button", "body", "MyView"] or at least contain these texts in some kind of structure so I can read them out. But it's completely empty for me. Am I missing something?

Steps to Reproduce

This is the helper function in my ExpressionMacro I'm trying to implement:

   /// Returns `MyView.body.Button.SaveChanges` for the following view:
   /// ```swift
   /// struct MyView: View {
   ///    var body: some View {
   ///       Button(#tk("Save Changes")) {
   ///          self.handleSave()
   ///       }
   ///    }
   /// }
   /// ```
   /// Or `MyModel.DisplayName.Movie.movie` and `MyModel.DisplayName.Series.tvShow` for the following model:
   /// ```swift
   /// enum MyModel: String, CaseIterable, Codable {
   ///    case movie
   ///    case series
   ///
   ///    var displayName: String {
   ///       switch self {
   ///       case .movie: #tk("Movie")
   ///       case .series: #tk("TV Show")
   ///       }
   ///    }
   /// }
   /// ```
   /// - NOTE: Use https://swift-ast-explorer.com for development.
   private static func semanticKey(
      of node: some FreestandingMacroExpansionSyntax,
      in context: some MacroExpansionContext
   ) -> String {
      // node-based approach
      return node.parent?.description ?? "N/A"  // always returns `"N/A"`

      // context-based approach
      return context.lexicalContext.description  // always returns `"[]"`
   }

[ahoppen]

Synced to Apple’s issue tracker as rdar://140829016

[ahoppen]

Are you seeing the empty lexical context when testing the macro using assertMacroExpansion and/or when expanding the macro during compilation?

[Jeehut]

I'm using the swift-macro-testing library of point-free in my tests, because when I wrote tests with assertMacroExpansion the tests never failed even when I had not written any code yet inside my expression macro implementation. I don't know if I did something wrong, but everything immediately worked as expected with their library.

But I did try it in a real-world project too, and the result was exactly the same as with the macro-testing library.

For your convenience, I attached a full minimal Swift package with which you can reproduce the exact problem I'm reporting here by simply running the tests. I've also added an assertMacroExpansion version for you, but like I said, they're pretty much useless because they're never failing:
TranslateKit-Issue-2907.zip

[ahoppen]

Thanks for providing the reproducing project. It looks like there are two independent issues:

1. In order to receive diagnostics from assertMacroExpansion when using swift-testing, you need to import SwiftSyntaxMacrosGenericTestSupport and provide a failureHandler as follows. With that, you see the correct lexical context (and some errors in your macro because context.lexicalContext.description is multiline and thus doesn’t fit into the single-line string literal that you created. https://github.com/swiftlang/swift-syntax/blob/main/Release%20Notes/600.md has some details about SwiftSyntaxMacrosGenericTestSupport and Adopt Swift-Testing in test utils such as SwiftSyntaxMacrosTestSupport #2720 is tracking that you shouldn’t need to specify a failure handler

failureHandler: { Issue.record("\($0.message)", sourceLocation: SourceLocation(fileID: $0.location.fileID, filePath: $0.location.filePath, line: $0.location.line, column: $0.location.column)) }

2. It appears that MacroTesting is not passing in the lexical context through. That sounds like an issue with the swift-macro-testing library, not with swift-syntax.

I’m going to close this issue because we are already tracking (1) and (2) is not an issue with swift-syntax itself. Feel free to re-open it if I missed anything.

[Jeehut]

@ahoppen Thank you for the quick response and the explanation!

After you said that the MacroTesting library is not passing the lexical context, I wanted to report it in their repo and found that somebody had already run into this and fixed it in PR pointfreeco/swift-macro-testing#31. So I switched the lib to the main branch and boom, everything works as expected!

Of course, returning context.lexicalContext.description wasn't quite right, but I didn't know what to return since I always got an empty array, so I didn't know what to work with. But now I'm getting a bunch of context, so I can finally try to see if I can auto-generate a useful localization key. Thank you for your quick help! 🙌