Swift Serialize Debugging Options

Overview

This is an undocumented Swift frontend flag. Neither invoking swift -help nor swift -help-hidden shows this flag (as of Xcode 12.0 beta). Update: This is documented in

$ swift -frontend -help

  -no-serialize-debugging-options
      Never serialize options for debugging (default: only for apps)
  -serialize-debugging-options
      Always serialize options for debugging (default: only for apps)

The equivalent build setting in Xcode is SWIFT_SERIALIZE_DEBUGGING_OPTIONS. Below is the information found in the Swift.xcspec file bundled within Xcode.

{
  Name = "SWIFT_SERIALIZE_DEBUGGING_OPTIONS";
  Type = Boolean;
  DefaultValue = YES;
  CommandLineArgs = {
    YES = ( "-Xfrontend", "-serialize-debugging-options" );
    NO  = ();
  };
},

This expands to -Xfrontend -serialize-debugging-options when passing to the Swift compiler. You could disable it by either setting

SWIFT_SERIALIZE_DEBUGGING_OPTIONS = NO

or

OTHER_SWIFT_FLAGS = -Xfrontend -no-serialize-debugging-options

in your target’s build settings (notice the -no prefix).

Discussions

When building with this flag enabled, the debugging information in the generated .swiftmodule will include the information of Swift module/framework search paths and extra Clang flags embedded as absolute paths.

If you build libraries or frameworks for third-party distribution, you should disable this flag in your release builds. It is reported that enabling it in the builds that you distribute to your users may break their debugging experience.

Enabling it also means that your builds will not be portable—you won’t be able to build your Swift module on a machine and debug it on another. If you use a build system that relies on cacheable builds like Bazel or Buck, you may want to disable this flag. To work around the debugging problem, you could manually reconstruct the search paths and extra Clang flags by editing your ~/.lldbinit file; for example:

settings set -- target.swift-extra-clang-flags <flags>
settings set -- target.swift-framework-search-paths <paths>
settings set -- target.swift-module-search-paths <paths>

Though, it may not work in all cases.

References