Swift 6.4 debug info tracks module identity precisely for faster expression evaluation

The change addresses a long-standing ambiguity in how LLDB resolves Swift modules during expression evaluation. Previously, LLDB relied on module names found in debug info, which could lead to incorrect module selection when multiple variants of a module existed, for example, private and public interfaces or platform-specific builds. The new approach embeds explicit references to the exact module used during compilation, eliminating this ambiguity.

For most developers, the improvement will be automatic. SwiftPM and Xcode projects will generate the updated debug info without any configuration changes, resulting in faster and more reliable debugging sessions. The performance gain comes from avoiding redundant module compilation when LLDB encounters a module name that could refer to multiple variants. This is particularly noticeable in large projects with complex dependency graphs.

Developers using custom build systems like Bazel, Buck, or CMake may need to update their toolchains to support the new debug info format. The article suggests that some build steps, such as those handling `-add_ast_path` linker flags, may no longer be necessary. However, the exact adjustments depend on how the build system interacts with the Swift compiler and linker, particularly on non-Darwin platforms where linker support for Swift modules is limited.

The change also has implications for debugging without dSYM bundles. On Darwin platforms, the linker embeds Swift module paths in the binary’s symbol table, which LLDB can read directly. The new debug info format preserves this capability while adding explicit module references, ensuring compatibility with existing workflows. However, the article does not detail how this will work on platforms where the linker lacks Swift-specific support, leaving some uncertainty for cross-platform projects.