Ensure that PCMs are hermetic on Xcode 16 and higher.

swift/internal/feature_names.bzl

@@ -132,6 +132,11 @@ SWIFT_FEATURE_FULL_DEBUG_INFO = "swift.full_debug_info"
 # Use CodeView debug information, which enables generation of PDBs for debugging.
 SWIFT_FEATURE_CODEVIEW_DEBUG_INFO = "swift.codeview_debug_info"
 
+# If enabled, paths embedded into precompiled Clang modules will be relative to
+# the workspace, not to the modulemap file location. Paths in the module maps
+# themselves will still be treated as modulemap-relative.
+SWIFT_FEATURE_MODULE_HOME_IS_CWD = "swift.module_home_is_cwd"
+
 # If enabled, compilation actions and module map generation will assume that the
 # header paths in module maps are relative to the current working directory
 # (i.e., the workspace root); if disabled, header paths in module maps are

swift/toolchains/config/compile_config.bzl

@@ -48,6 +48,7 @@ load(
     "SWIFT_FEATURE_DISABLE_SWIFT_SANDBOX",
     "SWIFT_FEATURE_DISABLE_SYSTEM_INDEX",
     "SWIFT_FEATURE_EMIT_BC",
+    "SWIFT_FEATURE_EMIT_C_MODULE",
     "SWIFT_FEATURE_EMIT_PRIVATE_SWIFTINTERFACE",
     "SWIFT_FEATURE_EMIT_SWIFTDOC",
     "SWIFT_FEATURE_EMIT_SWIFTINTERFACE",
@@ -67,6 +68,7 @@ load(
     "SWIFT_FEATURE_INTERNALIZE_AT_LINK",
     "SWIFT_FEATURE_LAYERING_CHECK",
     "SWIFT_FEATURE_MODULAR_INDEXING",
+    "SWIFT_FEATURE_MODULE_HOME_IS_CWD",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
     "SWIFT_FEATURE_NO_ASAN_VERSION_CHECK",
     "SWIFT_FEATURE_OPT",
@@ -658,6 +660,52 @@ def compile_action_configs(
             ],
         ),
 
+        # Explicitly set the working directory to ensure that the
+        # `FILE_SYSTEM_OPTIONS` block of PCM files is hermetic.
+        #
+        # IMPORTANT: When writing a PCM file, Clang *unconditionally* includes
+        # the working directory in the `FILE_SYSTEM_OPTIONS` block. Thus, the
+        # only way to ensure that these files are hermetic is to pass
+        # `-working-directory=.`. We cannot pass this to Swift, however, because
+        # the driver unfortunately resolves whatever path is given to it and
+        # then passes all of the source files as absolute paths to the
+        # frontend, which makes other outputs non-hermetic. Therefore, we *only*
+        # pass this flag to Clang. Having the two values not be literally
+        # identical should still be safe, because we're only passing a value
+        # here that is *effectively* the same as the default.
+        ActionConfigInfo(
+            actions = all_compile_action_names() + [
+                SWIFT_ACTION_PRECOMPILE_C_MODULE,
+            ],
+            configurators = [
+                add_arg("-Xcc", "-working-directory"),
+                add_arg("-Xcc", "."),
+            ],
+            features = [[
+                SWIFT_FEATURE_EMIT_C_MODULE,
+                SWIFT_FEATURE_USE_C_MODULES,
+                SWIFT_FEATURE_MODULE_HOME_IS_CWD,
+            ]],
+        ),
+        # Treat paths embedded into .pcm files as workspace-relative, not
+        # modulemap-relative.
+        ActionConfigInfo(
+            actions = all_compile_action_names() + [
+                SWIFT_ACTION_COMPILE_MODULE_INTERFACE,
+                SWIFT_ACTION_PRECOMPILE_C_MODULE,
+                SWIFT_ACTION_SYMBOL_GRAPH_EXTRACT,
+            ],
+            configurators = [
+                add_arg("-Xcc", "-Xclang"),
+                add_arg("-Xcc", "-fmodule-file-home-is-cwd"),
+            ],
+            features = [[
+                SWIFT_FEATURE_EMIT_C_MODULE,
+                SWIFT_FEATURE_USE_C_MODULES,
+                SWIFT_FEATURE_MODULE_HOME_IS_CWD,
+            ]],
+        ),
+
         # Treat paths in .modulemap files as workspace-relative, not modulemap-
         # relative.
         ActionConfigInfo(
@@ -1096,7 +1144,7 @@ def compile_action_configs(
                 SWIFT_ACTION_PRECOMPILE_C_MODULE,
             ],
             configurators = [
-                add_arg("-file-prefix-map", "__BAZEL_XCODE_DEVELOPER_DIR__=DEVELOPER_DIR"),
+                add_arg("-file-prefix-map", "__BAZEL_XCODE_DEVELOPER_DIR__=/PLACEHOLDER_DEVELOPER_DIR"),
             ],
             features = [SWIFT_FEATURE_FILE_PREFIX_MAP],
         ),

swift/toolchains/xcode_swift_toolchain.bzl

@@ -56,6 +56,7 @@ load(
     "SWIFT_FEATURE_DEBUG_PREFIX_MAP",
     "SWIFT_FEATURE_DISABLE_SWIFT_SANDBOX",
     "SWIFT_FEATURE_FILE_PREFIX_MAP",
+    "SWIFT_FEATURE_MODULE_HOME_IS_CWD",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
     "SWIFT_FEATURE_REMAP_XCODE_PATH",
     "SWIFT_FEATURE__SUPPORTS_UPCOMING_FEATURES",
@@ -708,7 +709,16 @@ def _xcode_swift_toolchain_impl(ctx):
         requested_features.append(SWIFT_FEATURE_DISABLE_SWIFT_SANDBOX)
 
     if _is_xcode_at_least_version(xcode_config, "16.0"):
-        requested_features.append(SWIFT_FEATURE__SUPPORTS_V6)
+        requested_features.extend([
+            SWIFT_FEATURE__SUPPORTS_V6,
+            # Ensure hermetic PCM files (no absolute workspace paths). This flag
+            # is actually supported on Xcode 15.x as well, but it's bugged; a
+            # `MODULE_DIRECTORY` field remains in the PCM file that has the
+            # absolute workspace path, so it's not hermetic, and worse, it
+            # causes other compilation errors in the unfortunately common case
+            # where two modules contain the same header.
+            SWIFT_FEATURE_MODULE_HOME_IS_CWD,
+        ])
 
     unsupported_features = ctx.disabled_features + [
         SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD,

tools/worker/swift_runner.cc

@@ -285,6 +285,24 @@ bool SwiftRunner::ProcessArgument(
     Iterator &itr, const std::string &arg,
     std::function<void(const std::string &)> consumer) {
   bool changed = false;
+
+  // Helper function for adding path remapping flags that depend on information
+  // only known at execution time.
+  auto add_prefix_map_flags = [&](const std::string &flag) {
+    // Get the actual current working directory (the execution root), which
+    // we didn't know at analysis time.
+    consumer(flag);
+    consumer(std::filesystem::current_path().string() + "=.");
+
+#if __APPLE__
+    std::string developer_dir = "__BAZEL_XCODE_DEVELOPER_DIR__";
+    if (bazel_placeholder_substitutions_.Apply(developer_dir)) {
+      consumer(flag);
+      consumer(developer_dir + "=/PLACEHOLDER_DEVELOPER_DIR");
+    }
+#endif
+  };
+
   if (arg[0] == '@') {
     changed = ProcessPossibleResponseFile(arg, consumer);
   } else {
@@ -293,20 +311,17 @@ bool SwiftRunner::ProcessArgument(
       if (new_arg == "-debug-prefix-pwd-is-dot") {
         // Replace the $PWD with . to make the paths relative to the workspace
         // without breaking hermiticity.
-        consumer("-debug-prefix-map");
-        consumer(std::filesystem::current_path().string() + "=.");
+        add_prefix_map_flags("-debug-prefix-map");
         changed = true;
       } else if (new_arg == "-coverage-prefix-pwd-is-dot") {
         // Replace the $PWD with . to make the paths relative to the workspace
         // without breaking hermiticity.
-        consumer("-coverage-prefix-map");
-        consumer(std::filesystem::current_path().string() + "=.");
+        add_prefix_map_flags("-file-prefix-map");
         changed = true;
       } else if (new_arg == "-file-prefix-pwd-is-dot") {
         // Replace the $PWD with . to make the paths relative to the workspace
         // without breaking hermiticity.
-        consumer("-file-prefix-map");
-        consumer(std::filesystem::current_path().string() + "=.");
+        add_prefix_map_flags("-file-prefix-map");
         changed = true;
       } else if (StripPrefix("-macro-expansion-dir=", new_arg)) {
         changed = true;