Add swift.use_tmpdir_for_module_cache feature

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

This feature, when enabled, will makes the Swift compilation actions use
the shared Clang module cache path written to
`/private/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

@@ -59,6 +59,7 @@ load(
     "SWIFT_FEATURE_SYSTEM_MODULE",
     "SWIFT_FEATURE_USE_C_MODULES",
     "SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE",
+    "SWIFT_FEATURE_USE_TMPDIR_FOR_MODULE_CACHE",
     "SWIFT_FEATURE_VFSOVERLAY",
 )
 load(":features.bzl", "are_all_features_enabled", "is_feature_enabled")
@@ -446,7 +447,19 @@ def compile_action_configs():
             ],
             configurators = [_global_module_cache_configurator],
             features = [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
-            not_features = [SWIFT_FEATURE_USE_C_MODULES],
+            not_features = [
+                SWIFT_FEATURE_USE_C_MODULES,
+                SWIFT_FEATURE_USE_TMPDIR_FOR_MODULE_CACHE,
+            ],
+        ),
+        swift_toolchain_config.action_config(
+            actions = [swift_action_names.COMPILE],
+            configurators = [_use_tmpdir_for_module_cache_configurator],
+            features = [
+                SWIFT_FEATURE_USE_C_MODULES,
+                SWIFT_FEATURE_USE_TMPDIR_FOR_MODULE_CACHE,
+            ],
+            not_features = [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
         ),
         swift_toolchain_config.action_config(
             actions = [
@@ -461,6 +474,7 @@ def compile_action_configs():
             not_features = [
                 [SWIFT_FEATURE_USE_C_MODULES],
                 [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
+                [SWIFT_FEATURE_USE_TMPDIR_FOR_MODULE_CACHE],
             ],
         ),
         swift_toolchain_config.action_config(
@@ -818,6 +832,18 @@ def _global_module_cache_configurator(prerequisites, args):
             paths.join(prerequisites.bin_dir.path, "_swift_module_cache"),
         )
 
+def _use_tmpdir_for_module_cache_configurator(prerequisites, args):
+    """Adds flags to use a pre-defined module cache path."""
+    if prerequisites.bin_dir:
+        args.add(
+            "-module-cache-path",
+            paths.join(
+                "/private/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

@@ -155,6 +155,20 @@ 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 `/private/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_USE_TMPDIR_FOR_MODULE_CACHE = "swift.use_tmpdir_for_module_cache"
+
 # 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