Add feature to prevent embedded build paths in swiftmodules

swift/internal/api.bzl

@@ -262,12 +262,22 @@ def _compilation_mode_copts(feature_configuration):
             flags.append("-whole-module-optimization")
 
     elif is_dbg or is_fastbuild:
-        # The Swift compiler only serializes debugging options in narrow
-        # circumstances (for example, for application binaries). Since we almost
-        # exclusively just compile to object files directly, we need to manually
-        # pass the following frontend option to ensure that LLDB has the necessary
-        # import search paths to find definitions during debugging.
-        flags.extend(["-Onone", "-DDEBUG", "-Xfrontend", "-serialize-debugging-options"])
+        flags.extend(["-Onone", "-DDEBUG"])
+        if _is_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE_CACHEABLE_SWIFTMODULES,
+        ):
+            # For a swiftmodule to be usable from cache, it must not contain embedded paths. This
+            # behavior must be explicitly disabled, because swiftc will enable this by default under
+            # some circumstances.
+            flags.extend(["-Xfrontend", "-no-serialize-debugging-options"])
+        else:
+            # The Swift compiler only serializes debugging options in narrow
+            # circumstances (for example, for application binaries). Since we almost
+            # exclusively just compile to object files directly, we need to manually
+            # pass the following frontend option to ensure that LLDB has the necessary
+            # import search paths to find definitions during debugging.
+            flags.extend(["-Xfrontend", "-serialize-debugging-options"])
 
     if _is_enabled(
         feature_configuration = feature_configuration,

swift/internal/features.bzl

@@ -105,6 +105,15 @@ SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE = "swift.use_global_module_cache"
 # typically set this automatically if using a sufficiently recent version of Swift (4.2 or higher).
 SWIFT_FEATURE_USE_RESPONSE_FILES = "swift.use_response_files"
 
+# If enabled, builds using the "dbg" compilation mode will explicitly disable swiftc from producing
+# swiftmodules containing embedded file paths, which are inherently non-portable across machines.
+#
+# To used these modules from lldb, target settings must be correctly populated. For example:
+#     target.swift-module-search-paths
+#     target.swift-framework-search-paths
+#     target.swift-extra-clang-flags
+SWIFT_FEATURE_CACHEABLE_SWIFTMODULES = "swift.cacheable_swiftmodules"
+
 def features_for_build_modes(ctx, objc_fragment = None):
     """Returns a list of Swift toolchain features corresponding to current build modes.