Lazy prepare-for-indexing produces incomplete swiftmodules when types receive macro-synthesized extension conformances

[ktoso]

Swift version

swiftlang-6.4.0.19.4

Platform

all platforms

Editor

VSCode

Description

When background indexing runs with the default mode, modules that contain types decorated with macros that use @attached(extension, conformances: ...) appear to produce swiftmodules whose conformance information is incomplete or missing. Cross-module consumers then see spurious "type does not conform to protocol" diagnostics, broken completion, and broken jump-to-definition for APIs that depend on those conformances.

Switching to backgroundPreparationMode: "noLazy" seems to resolve the issues.

Previously thought to just expose this setting swiftlang/vscode-swift#2291 but instead we should fix the indexing.

Steps to Reproduce

swiftlang/swift-java editing experience is broken due to this and reproduces easily.

Additional context

swift-java's @JavaClass / @JavaInterface macros are declared as:

  @attached(member, names: …)
  @attached(extension, conformances: AnyJavaObject)
  public macro JavaClass(_ fullClassName: String, …)

Their ExtensionMacro expansion emits:

  extension <Type>: AnyJavaObject { }

These macros are applied to many types across many modules in the project, and downstream modules use those types where an AnyJavaObject conformance is required.

With backgroundPreparationMode: "enabled" (default), opening the project in VS Code with background indexing enabled produces sporadic but consistent failures (false errors, missing completion, broken jump-to-def) on code that should bevalid. With backgroundPreparationMode: "noLazy", everything works correctly.

The workaround currently committed to swift-java is .sourcekit-lsp/config.json: {"backgroundPreparationMode": "noLazy"}

Steps to reproduce

1. git clone https://github.com/swiftlang/swift-java && cd swift-java
2. rm .sourcekit-lsp/config.json to remove the workaround setting the noLazy mode
3. Open in VS Code with the Swift extension and background indexing enabled
4. Open files that import modules containing @JavaClass / @JavaInterface types, any file containing them diagnostics / broken IDE features on uses of those types

Logging

No response

[ktoso]

The actual bug may be in swiftc... I'm digging some more into this.