Add a feature to allow Swift targets to skip embedding the .swiftmodule file in debug info.

* To disable `.swiftmodule` embedding for a single `swift_{binary,library,test}` target, add `features = ["swift.no_embed_debug_module"]` to that target.
* To disable `.swiftmodule` embedding for all Swift targets in a build package, add `features = ["swift.no_embed_debug_module"]` to the `package(...)` declaration in that BUILD file.
* To disable `.swiftmodule` embedding for an entire build invocation, pass `--features=swift.no_embed_debug_module` on the command line.

(The feature only affects the target it is applied to, not its `deps`. To turn it off universally, use the command line option.)

This is intended to be a workaround for apps with large Swift dependency graphs until Xcode 12 releases a fixed version of `ld64` that supports param files. Using this feature may cause a regression in the debugging experience (e.g., breakpoints, expression evaluation) for modules where it is enabled.

RELNOTES: None.
PiperOrigin-RevId: 322195872

Author: allevato

swift/internal/compiling.bzl

@@ -45,6 +45,7 @@ load(
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
     "SWIFT_FEATURE_MINIMAL_DEPS",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
+    "SWIFT_FEATURE_NO_EMBED_DEBUG_MODULE",
     "SWIFT_FEATURE_NO_GENERATED_HEADER",
     "SWIFT_FEATURE_NO_GENERATED_MODULE_MAP",
     "SWIFT_FEATURE_OPT",
@@ -1704,7 +1705,14 @@ def _register_post_compile_actions(
     # binary for debugging purposes.
     linker_flags = []
     linker_inputs = []
-    if _is_debugging(feature_configuration = feature_configuration):
+    should_embed_swiftmodule_for_debugging = (
+        _is_debugging(feature_configuration = feature_configuration) and
+        not is_feature_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE_NO_EMBED_DEBUG_MODULE,
+        )
+    )
+    if should_embed_swiftmodule_for_debugging:
         module_embed_results = ensure_swiftmodule_is_embedded(
             actions = actions,
             feature_configuration = feature_configuration,

swift/internal/feature_names.bzl

@@ -191,3 +191,12 @@ SWIFT_FEATURE_EMIT_SWIFTINTERFACE = "swift.emit_swiftinterface"
 # This allows Bazel to avoid propagating swiftmodules of such dependencies
 # higher in the dependency graph than they need to be.
 SWIFT_FEATURE_SUPPORTS_PRIVATE_DEPS = "swift.supports_private_deps"
+
+# If enabled, the .swiftmodule file for the affected target will not be
+# embedded in debug info and propagated to the linker.
+#
+# The name of this feature is negative because it is meant to be a temporary
+# workaround until ld64 is fixed (in Xcode 12) so that builds that pass large
+# numbers of `-Wl,-add_ast_path,<path>` flags to the linker do not overrun the
+# system command line limit.
+SWIFT_FEATURE_NO_EMBED_DEBUG_MODULE = "swift.no_embed_debug_module"