Disable layering checks when compiling the explicit module for a Swift generated header.

The Swift compiler determines which module to import a symbol from based on the module that defines that symbol. Due to modular re-exports (which are particularly common among system frameworks), this may not be the same framework that the user imported from Swift and added to their dependencies.

(For example, most users will import and depend on `Foundation` to use `NSObject`, but Swift will import it from the module it is actually defined in, `ObjectiveC`.)

PiperOrigin-RevId: 371919747

Author: allevato

swift/internal/compiling.bzl

@@ -362,6 +362,21 @@ def compile_action_configs(
     ]
 
     #### Flags controlling how Swift/Clang modular inputs are processed
+
+    def c_layering_check_configurator(prerequisites, args, *, strict):
+        # We do not enforce layering checks for the Objective-C header generated
+        # by Swift, because we don't have predictable control over the imports
+        # that it generates. Due to modular re-exports (which are especially
+        # common among system frameworks), it may generate an import declaration
+        # for a particular symbol from a different module than the Swift code
+        # imported it from.
+        if not prerequisites.is_swift_generated_header:
+            args.add(
+                "-Xcc",
+                "-fmodules-strict-decluse" if strict else "-fmodules-decluse",
+            )
+        return None
+
     action_configs += [
         # Treat paths in .modulemap files as workspace-relative, not modulemap-
         # relative.
@@ -443,12 +458,10 @@ def compile_action_configs(
         swift_toolchain_config.action_config(
             actions = [swift_action_names.PRECOMPILE_C_MODULE],
             configurators = [
-                # Enforce `use` declarations for user modules since we generate
-                # those, but not for system modules since they typically do not
-                # have the proper `use` decls.
-                swift_toolchain_config.add_arg(
-                    "-Xcc",
-                    "-fmodules-decluse",
+                lambda prerequisites, args: c_layering_check_configurator(
+                    prerequisites,
+                    args,
+                    strict = False,
                 ),
             ],
             not_features = [
@@ -459,9 +472,10 @@ def compile_action_configs(
         swift_toolchain_config.action_config(
             actions = [swift_action_names.PRECOMPILE_C_MODULE],
             configurators = [
-                swift_toolchain_config.add_arg(
-                    "-Xcc",
-                    "-fmodules-strict-decluse",
+                lambda prerequisites, args: c_layering_check_configurator(
+                    prerequisites,
+                    args,
+                    strict = True,
                 ),
             ],
             features = [SWIFT_FEATURE_LAYERING_CHECK],
@@ -1480,14 +1494,15 @@ def compile(
         feature_configuration = feature_configuration,
         feature_name = SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,
     ):
-        precompiled_module = precompile_clang_module(
+        precompiled_module = _precompile_clang_module(
             actions = actions,
             bin_dir = bin_dir,
             cc_compilation_context = cc_common.create_compilation_context(
                 headers = depset([compile_outputs.generated_header_file]),
             ),
             feature_configuration = feature_configuration,
             genfiles_dir = genfiles_dir,
+            is_swift_generated_header = True,
             module_map_file = compile_outputs.generated_module_map_file,
             module_name = module_name,
             swift_info = create_swift_info(
@@ -1574,6 +1589,74 @@ def precompile_clang_module(
         swift_info: A `SwiftInfo` provider that contains dependencies required
             to compile this module.
 
+    Returns:
+        A `File` representing the precompiled module (`.pcm`) file, or `None` if
+        the toolchain or target does not support precompiled modules.
+    """
+    return _precompile_clang_module(
+        actions = actions,
+        bin_dir = bin_dir,
+        cc_compilation_context = cc_compilation_context,
+        feature_configuration = feature_configuration,
+        genfiles_dir = genfiles_dir,
+        is_swift_generated_header = False,
+        module_map_file = module_map_file,
+        module_name = module_name,
+        swift_info = swift_info,
+        swift_toolchain = swift_toolchain,
+        target_name = target_name,
+    )
+
+def _precompile_clang_module(
+        *,
+        actions,
+        cc_compilation_context,
+        feature_configuration,
+        is_swift_generated_header,
+        module_map_file,
+        module_name,
+        swift_toolchain,
+        target_name,
+        bin_dir = None,
+        genfiles_dir = None,
+        swift_info = None):
+    """Precompiles an explicit Clang module that is compatible with Swift.
+
+    Args:
+        actions: The context's `actions` object.
+        cc_compilation_context: A `CcCompilationContext` that contains headers
+            and other information needed to compile this module. This
+            compilation context should contain all headers required to compile
+            the module, which includes the headers for the module itself *and*
+            any others that must be present on the file system/in the sandbox
+            for compilation to succeed. The latter typically refers to the set
+            of headers of the direct dependencies of the module being compiled,
+            which Clang needs to be physically present before it detects that
+            they belong to one of the precompiled module dependencies.
+        feature_configuration: A feature configuration obtained from
+            `swift_common.configure_features`.
+        is_swift_generated_header: If True, the action is compiling the
+            Objective-C header generated by the Swift compiler for a module.
+        module_map_file: A textual module map file that defines the Clang module
+            to be compiled.
+        module_name: The name of the top-level module in the module map that
+            will be compiled.
+        swift_toolchain: The `SwiftToolchainInfo` provider of the toolchain.
+        target_name: The name of the target for which the code is being
+            compiled, which is used to determine unique file paths for the
+            outputs.
+        bin_dir: The Bazel `*-bin` directory root. If provided, its path is used
+            to store the cache for modules precompiled by Swift's ClangImporter,
+            and it is added to ClangImporter's header search paths for
+            compatibility with Bazel's C++ and Objective-C rules which support
+            includes of generated headers from that location.
+        genfiles_dir: The Bazel `*-genfiles` directory root. If provided, its
+            path is added to ClangImporter's header search paths for
+            compatibility with Bazel's C++ and Objective-C rules which support
+            inclusions of generated headers from that location.
+        swift_info: A `SwiftInfo` provider that contains dependencies required
+            to compile this module.
+
     Returns:
         A `File` representing the precompiled module (`.pcm`) file, or `None` if
         the toolchain or target does not support precompiled modules.
@@ -1608,6 +1691,7 @@ def precompile_clang_module(
         cc_info = CcInfo(compilation_context = cc_compilation_context),
         genfiles_dir = genfiles_dir,
         is_swift = False,
+        is_swift_generated_header = is_swift_generated_header,
         module_name = module_name,
         objc_include_paths_workaround = depset(),
         objc_info = apple_common.new_objc_provider(),