Debuggable Remotely Built Swift

This is a guide to using remotely built Swift modules in local debug builds.

At the time of writing, lldb depends on debugging options embedded in .swiftmodule files. These options include paths that are only valid on the build host. For local builds, this all just works, but for remote builds, it doesn't.

The solution is two parts:

  1. Use -no-serialize-debugging-options globally, to prevent embedded paths
  2. Use -serialize-debugging-options locally in one empty module

Globally disabling debugging options makes those .swiftmodules usable on any machine. Locally enabling debugging options for one module provides lldb with enough info to make debugging work.

An lldb bug has been filed here:

Disable Debugging Options Globally

To globally disable debugging options, use the swift.cacheable_swiftmodules feature in rules_swift. For example, your .bazelrc could look like this:

build --features=swift.cacheable_swiftmodules

What this does is ensure all modules are built explicitly with -no-serialize-debugging-options. It has to be explicit because swiftc enables -serialize-debugging-options in some cases.

Add Debug Build Config

In a BUILD file - in this example, the root BUILD file, define the following config_setting. This will allow targets to conditionally depend on the locally built debugging module.

    name = "debug",
    values = {
        "compilation_mode": "dbg",

Define Local Debug Target

In the BUILD file of your choice, define a swift_library with:

  • A single empty source file
  • Enables -serialize-debugging-options
  • Built locally, not remote - using the no-remote tag

Here is one way to define the BUILD file, using a genrule to create the empty swift file.

    name = "empty",
    outs = ["empty.swift"],
    cmd = "touch $(OUTS)",

    name = "_LocalDebugOptions",
    srcs = [":empty"],
    copts = [
    module_name = "_LocalDebugOptions",
    tags = ["no-remote"],
    visibility = ["//visibility:public"],

Update Top Level Test Targets

Finally, for each top-level test target (ios_unit_test, ios_ui_test*, etc), conditionally add the local debugging module to the deps. This is done via the debug config. In the past this also had to be done for ios_application targets but that has since been fixed in lldb.

debug_deps = select({
    "//:debug": ["//some/path:_LocalDebugOptions"],
    "//conditions:default": [],

    name = "...",
    deps = debug_deps + [
        # ...
    # ...
Note about ios_unit_test

When using a test host, the debugging module must be added to the test host target only, not the unit test target. However, for tests without a test host, the debugging module must be added to the unit test target.

LLDB Settings

Additional settings may be required, depending on your build setup. For example, an Xcode Run Script may look like:

echo "settings set target.sdk-path $SDKROOT"
echo "settings set target.swift-framework-search-paths $FRAMEWORK_SEARCH_PATHS"

Other settings you can try customizing are:

  • target.clang-module-search-paths
  • target.debug-file-search-paths
  • target.sdk-path
  • target.swift-extra-clang-flags
  • target.swift-framework-search-paths
  • target.swift-module-search-paths
  • target.use-all-compiler-flags
  • symbols.clang-modules-cache-path

These settings would be written to some project specific lldbinit file which you can include directly in Xcode's scheme.