Swift: don't show non-user triggered warning for serialized debugging…

… options

Summary: Ensure we don't trigger `serialize_debugging_options` warnings unnecessarily.

Reviewed By: manicaesar

Differential Revision: D63316586

fbshipit-source-id: d9f1d8763dc654baa495382a038f86fbf8a5ee66

apple/apple_common.bzl

@@ -155,7 +155,13 @@ def _debug_artifacts_validators_arg():
 
 def _serialize_debugging_options_arg():
     return {
-        "serialize_debugging_options": attrs.bool(default = True),
+        # Need ability to distinguish between no value provided by users
+        # vs value explicitly set to `True` (in the latter case, we should
+        # show warning if value cannot be respected in mixed modules while
+        # in the former, we do not show a warning).
+        #
+        # Lack of value defaults to enabling serialized debugging options.
+        "serialize_debugging_options": attrs.option(attrs.bool(), default = None),
     }
 
 def _uses_explicit_modules_arg():

apple/swift/swift_compilation.bzl

@@ -578,6 +578,11 @@ def _compile_with_argsfile(
     # Swift correctly handles relative paths and we can utilize the relative argsfile for Xcode.
     return CompileArgsfiles(relative = {extension: argsfile}, xcode = {extension: argsfile})
 
+def _get_serialize_debugging_options_attr_value(ctx: AnalysisContext):
+    if ctx.attrs.serialize_debugging_options == None:
+        return True
+    return ctx.attrs.serialize_debugging_options
+
 def _get_shared_flags(
         ctx: AnalysisContext,
         deps_providers: list,
@@ -657,7 +662,7 @@ def _get_shared_flags(
         else:
             cmd.add(["-enable-experimental-cxx-interop"])
 
-    serialize_debugging_options = ctx.attrs.serialize_debugging_options and (not explicit_modules_enabled) and (not objc_headers) and toolchain.prefix_serialized_debugging_options
+    serialize_debugging_options = _get_serialize_debugging_options_attr_value(ctx) and (not explicit_modules_enabled) and (not objc_headers) and toolchain.prefix_serialized_debugging_options
     if serialize_debugging_options:
         cmd.add([
             "-Xfrontend",