Persist pch files in output dir (bazelbuild#701)

swift/internal/compiling.bzl

@@ -60,6 +60,7 @@ load(
     "SWIFT_FEATURE_USE_C_MODULES",
     "SWIFT_FEATURE_USE_GLOBAL_INDEX_STORE",
     "SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE",
+    "SWIFT_FEATURE_USE_PCH_OUTPUT_DIR",
     "SWIFT_FEATURE_VFSOVERLAY",
     "SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS",
     "SWIFT_FEATURE__WMO_IN_SWIFTCOPTS",
@@ -572,6 +573,18 @@ def compile_action_configs(
                 [SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE],
             ],
         ),
+        swift_toolchain_config.action_config(
+            actions = [
+                swift_action_names.COMPILE,
+                swift_action_names.DERIVE_FILES,
+                swift_action_names.DUMP_AST,
+            ],
+            configurators = [_pch_output_dir_configurator],
+            features = [
+                SWIFT_FEATURE_USE_PCH_OUTPUT_DIR,
+            ],
+        ),
+
         # When using C modules, disable the implicit search for module map files
         # because all of them, including system dependencies, will be provided
         # explicitly.
@@ -1491,6 +1504,21 @@ def _index_while_building_configurator(prerequisites, args):
     if not _index_store_path_overridden(prerequisites.user_compile_flags):
         args.add("-index-store-path", prerequisites.indexstore_directory.path)
 
+def _pch_output_dir_configurator(prerequisites, args):
+    """Adds flags for pch-output-dir configuration to the command line.
+
+      This is a directory to persist automatically created precompiled bridging headers
+
+      Note: that like the global index store and module cache, we expect clang
+      to namespace these correctly per arch / os version / etc by the hash in
+      the path. However, it is also put into the bin_dir for an added layer of
+      safety.
+    """
+    args.add(
+        "-pch-output-dir",
+        paths.join(prerequisites.bin_dir.path, "_pch_output_dir"),
+    )
+
 def _global_index_store_configurator(prerequisites, args):
     """Adds flags for index-store generation to the command line."""
     out_dir = prerequisites.indexstore_directory.dirname.split("/")[0]

swift/internal/feature_names.bzl

@@ -270,3 +270,7 @@ SWIFT_FEATURE__WMO_IN_SWIFTCOPTS = "swift._wmo_in_swiftcopts"
 # were passed on the command line using `--swiftcopt`. Users should never
 # manually enable, disable, or query this feature.
 SWIFT_FEATURE__NUM_THREADS_0_IN_SWIFTCOPTS = "swift._num_threads_0_in_swiftcopts"
+
+# A feature to enable setting pch-output-dir
+# This is a directory to persist automatically created precompiled bridging headers
+SWIFT_FEATURE_USE_PCH_OUTPUT_DIR = "swift.use_pch_output_dir"

test/BUILD

@@ -8,6 +8,7 @@ load(":output_file_map_tests.bzl", "output_file_map_test_suite")
 load(":private_deps_tests.bzl", "private_deps_test_suite")
 load(":swift_through_non_swift_tests.bzl", "swift_through_non_swift_test_suite")
 load(":split_derived_files_tests.bzl", "split_derived_files_test_suite")
+load(":pch_output_dir_tests.bzl", "pch_output_dir_test_suite")
 
 licenses(["notice"])
 
@@ -29,6 +30,8 @@ swift_through_non_swift_test_suite()
 
 split_derived_files_test_suite()
 
+pch_output_dir_test_suite()
+
 test_suite(
     name = "all_tests",
 )

test/pch_output_dir_tests.bzl

@@ -0,0 +1,54 @@
+# Copyright 2021 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Tests for pch output dir command line flags"""
+
+load(
+    "@build_bazel_rules_swift//test/rules:action_command_line_test.bzl",
+    "make_action_command_line_test_rule",
+)
+
+pch_output_dir_action_command_line_test = make_action_command_line_test_rule(
+    config_settings = {
+        "//command_line_option:features": [
+            "swift.use_pch_output_dir",
+        ],
+    },
+)
+
+def pch_output_dir_test_suite(name = "pch_output_dir_settings"):
+    """Test suite for pch output dir options.
+
+    Args:
+        name: The name prefix for all the nested tests
+    """
+
+    # Verify that a pch dir is passed
+    pch_output_dir_action_command_line_test(
+        name = "{}_pch_output_dir".format(name),
+        expected_argv = [
+            "-pch-output-dir",
+            # Starlark doesn't have support for regular expression yet, so we
+            # can't verify the whole argument here. It has the configuration
+            # fragment baked in.
+        ],
+        mnemonic = "SwiftCompile",
+        tags = [name],
+        target_under_test = "@build_bazel_rules_swift//test/fixtures/debug_settings:simple",
+    )
+
+    native.test_suite(
+        name = name,
+        tags = [name],
+    )