Add swift.deterministic_module_breadcrumbs feature

This adds a new feature, that can be enabled by passing
`--features=swift.deterministic_module_breadcrumbs` to the build
command.

This feature, when enabled, will makes the Swift compilation actions use
the shared Clang module cache path written to
`/tmp/build_bazel_rules_swift`. This makes the embedded Clang module
breadcrumbs deterministic between Bazel instances, because they are
always embedded as absolute paths. Note that the use of this cache is
non-hermetic--the cached modules are not wiped between builds, and won't
be cleaned when invoking `bazel clean`; the user is responsible for
manually cleaning them.

Additionally, this can be used as a workaround for a bug in the Swift
compiler that causes the module breadcrumbs to be embedded even though the
`-no-clang-module-breadcrumbs` flag is passed
(https://bugs.swift.org/browse/SR-13275).

Author: thii

swift/internal/compiling.bzl

@@ -34,6 +34,7 @@ load(
     "SWIFT_FEATURE_COVERAGE",
     "SWIFT_FEATURE_DBG",
     "SWIFT_FEATURE_DEBUG_PREFIX_MAP",
+    "SWIFT_FEATURE_DETERMINISTIC_MODULE_BREADCRUMBS",
     "SWIFT_FEATURE_EMIT_C_MODULE",
     "SWIFT_FEATURE_EMIT_SWIFTINTERFACE",
     "SWIFT_FEATURE_ENABLE_BATCH_MODE",
@@ -352,6 +353,16 @@ def compile_action_configs():
                 SWIFT_FEATURE_IMPLICIT_MODULES,
                 SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE,
             ],
+            not_features = [SWIFT_FEATURE_DETERMINISTIC_MODULE_BREADCRUMBS],
+        ),
+        swift_toolchain_config.action_config(
+            actions = [swift_action_names.COMPILE],
+            configurators = [_deterministic_module_breadcrumbs_configurator],
+            features = [
+                SWIFT_FEATURE_IMPLICIT_MODULES,
+                SWIFT_FEATURE_DETERMINISTIC_MODULE_BREADCRUMBS,
+            ],
+            not_features = [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
         ),
         swift_toolchain_config.action_config(
             actions = [swift_action_names.COMPILE],
@@ -361,7 +372,10 @@ def compile_action_configs():
                 ),
             ],
             features = [SWIFT_FEATURE_IMPLICIT_MODULES],
-            not_features = [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
+            not_features = [
+                [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
+                [SWIFT_FEATURE_DETERMINISTIC_MODULE_BREADCRUMBS],
+            ],
         ),
     ]
 
@@ -592,6 +606,19 @@ def _global_module_cache_configurator(prerequisites, args):
             paths.join(prerequisites.bin_dir.path, "_swift_module_cache"),
         )
 
+def _deterministic_module_breadcrumbs_configurator(prerequisites, args):
+    """Adds flags to use a hard-coded module cache path. This makes the Clang
+    module breadcrumbs embedded in the produced object file deterministic."""
+    if prerequisites.bin_dir:
+        args.add(
+            "-module-cache-path",
+            paths.join(
+                "/tmp/build_bazel_rules_swift",
+                prerequisites.bin_dir.path,
+                "_swift_module_cache",
+            ),
+        )
+
 def _batch_mode_configurator(prerequisites, args):
     """Adds flags to enable batch compilation mode."""
     if not _is_wmo_manually_requested(prerequisites.user_compile_flags):

swift/internal/feature_names.bzl

@@ -148,6 +148,20 @@ SWIFT_FEATURE_USE_C_MODULES = "swift.use_c_modules"
 # crashes.
 SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE = "swift.use_global_module_cache"
 
+# If enabled, Swift compilation actions will use the shared Clang module cache
+# path written to `/tmp/build_bazel_rules_swift`. This makes the embedded Clang
+# module breadcrumbs deterministic between Bazel instances, because they are
+# always embedded as absolute paths. Note that the use of this cache is
+# non-hermetic--the cached modules are not wiped between builds, and won't be
+# cleaned when invoking `bazel clean`; the user is responsible for manually
+# cleaning them.
+#
+# Additionally, this can be used as a workaround for a bug in the Swift
+# compiler that causes the module breadcrumbs to be embedded even though the
+# `-no-clang-module-breadcrumbs` flag is passed
+# (https://bugs.swift.org/browse/SR-13275).
+SWIFT_FEATURE_DETERMINISTIC_MODULE_BREADCRUMBS = "swift.deterministic_module_breadcrumbs"
+
 # If enabled, actions invoking the Swift driver or frontend may write argument
 # lists into response files (i.e., "@args.txt") to avoid passing command lines
 # that exceed the system limit. Toolchains typically set this automatically if