No diagnostics is produced for non-default traits

[MaxDesiatov]

Swift version

swift-DEVELOPMENT-SNAPSHOT-2025-11-03-a, also reproducible with Xcode 26.0.1.

Platform

macOS 15.7.2

Editor

Neovim

Description

No diagnostics is produced within code-blocks hidden in non-default traits even if such traits are enabled in .sourcekit-lsp/config.json. Code in such blocks is also not indexed.

Steps to Reproduce

mkdir traits-repro
cd traits-repro
swift package init

Make sure that Package.swift looks like this:

// swift-tools-version: 6.2

import PackageDescription

let package = Package(
    name: "traits-repro",
    products: [
        .library(
            name: "traits-repro",
            targets: ["traits-repro"]
        ),
    ],
    traits: [
        .default(enabledTraits: []),
        "NonDefaultTrait",
    ],
    targets: [
        .target(
            name: "traits-repro"
        ),
    ]
)

Enable the trait in .sourcekit-lsp/config.json:

{
    "swiftPM": {
        "traits": ["NonDefaultTrait"]
    }
}

Add following code to Sources/traits-repro/traits_repro.swift:

#if NonDefaultTrait

func f(arg: Int) {
    arg
}

func g() {
    f(arg: "foo")
}

#endif

You will see that no error diagnostic is produced in the body of func g() declaration as long as NonDefaultTrait is not included in the default trait.

Logging

No response

[ahoppen]

There are two issues here:

1. When background indexing is disabled, we don’t pass the trait configuration here
   

   sourcekit-lsp/Sources/BuildServerIntegration/SwiftPMBuildServer.swift

   Lines 444 to 455 in 5fe1079

   	let plan = try await BuildPlan(
   	destinationBuildParameters: destinationBuildParameters,
   	toolsBuildParameters: toolsBuildParameters,
   	graph: modulesGraph,
   	disableSandbox: options.swiftPMOrDefault.disableSandbox ?? false,
   	fileSystem: localFileSystem,
   	observabilityScope: observabilitySystem.topScope.makeChildScope(description: "Create SwiftPM build plan")
   	)
   	signposter.emitEvent("Finished generating build plan", id: signpostID)
   	buildDescription = BuildDescription(buildPlan: plan)

2. When background indexing is enabled, we seem to loose the trait configuration somewhere. I debugged a little bit into SwiftPM and suspect that some code path in SourceKitLSPAPI just doesn’t match the normal package loading logic.

Here’s a reduced test case that reproduces this issue, it should pass both with enableBackgroundIndexing: true and enableBackgroundIndexing: false

func testPackagePlugin() async throws {
  let testProject = try await SwiftPMTestProject(
    files: [
      "Test.swift": """
      #if NonDefaultTrait
      #warning("Trait enabled")
      #endif
      """
    ],
    manifest: """
      // swift-tools-version: 6.2

      import PackageDescription

      let package = Package(
        name: "MyLibrary",
        traits: [
          .default(enabledTraits: []),
          "NonDefaultTrait",
        ],
        targets: [.target(name: "MyLibrary")]
      )
      """,
    options: SourceKitLSPOptions(swiftPM: .init(traits: ["NonDefaultTrait"])),
    enableBackgroundIndexing: true
  )

  let (uri, _) = try testProject.openDocument("Test.swift")
  let diagnostics = try await testProject.testClient.send(
    DocumentDiagnosticsRequest(textDocument: TextDocumentIdentifier(uri))
  )
  XCTAssertEqual(diagnostics.fullReport?.items.map(\.message), ["Trait enabled"])
}

[MaxDesiatov]

cc @bripeticca