Add a feature to always include all headers as inputs to SwiftCompile…

… actions, even when using explicit modules

When building with explicit modules, Swift compilations do not require any of
the headers to be present on disk, because the AST contains all the necessary
information (save for a bug(?) in `canImport` which fails if the headers aren't
also present). However, other tooling may want to inspect compilation
actions and have access to those headers.

This feature is a no-op for implicit module builds, since the headers are always
included to begin with, and it is off by default.

PiperOrigin-RevId: 461668350
(cherry picked from commit 81f7b54)
Signed-off-by: Brentley Jones <github@brentleyjones.com>

swift/internal/compiling.bzl

@@ -38,6 +38,7 @@ load(
     "SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO",
     "SWIFT_FEATURE_ENABLE_LIBRARY_EVOLUTION",
     "SWIFT_FEATURE_FULL_LTO",
+    "SWIFT_FEATURE_HEADERS_ALWAYS_ACTION_INPUTS",
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
     "SWIFT_FEATURE_NO_GENERATED_MODULE_MAP",
     "SWIFT_FEATURE_OPT",
@@ -662,6 +663,10 @@ to use swift_common.compile(include_dev_srch_paths = ...) instead.\
 
     prerequisites = struct(
         additional_inputs = additional_inputs,
+        always_include_headers = is_feature_enabled(
+            feature_configuration = feature_configuration,
+            feature_name = SWIFT_FEATURE_HEADERS_ALWAYS_ACTION_INPUTS,
+        ),
         bin_dir = feature_configuration._bin_dir,
         cc_compilation_context = merged_cc_info.compilation_context,
         const_protocols_to_gather_file = const_protocols_to_gather_file,

swift/internal/feature_names.bzl

@@ -30,6 +30,10 @@ SWIFT_FEATURE_DBG = "swift.dbg"
 SWIFT_FEATURE_FASTBUILD = "swift.fastbuild"
 SWIFT_FEATURE_OPT = "swift.opt"
 
+# If True, transitive C headers will be always be passed as inputs to Swift
+# compilation actions, even when building with explicit modules.
+SWIFT_FEATURE_HEADERS_ALWAYS_ACTION_INPUTS = "swift.headers_always_action_inputs"
+
 # This feature is enabled if coverage collection is enabled for the build. (See
 # the note above about not depending on the C++ features.)
 SWIFT_FEATURE_COVERAGE = "swift.coverage"

swift/toolchains/config/compile_config.bzl

@@ -1363,12 +1363,16 @@ def _dependencies_clang_defines_configurator(prerequisites, args):
     args.add_all(all_clang_defines, before_each = "-Xcc", format_each = "-D%s")
 
 def _collect_clang_module_inputs(
+        always_include_headers,
         explicit_module_compilation_context,
         modules,
         prefer_precompiled_modules):
     """Collects Clang module-related inputs to pass to an action.
 
     Args:
+        always_include_headers: If True, pass the transitive headers as inputs
+            to the compilation action, even if doing an explicit module
+            compilation.
         explicit_module_compilation_context: The `CcCompilationContext` of the
             target being compiled, if the inputs are being collected for an
             explicit module compilation action. This parameter should be `None`
@@ -1417,12 +1421,16 @@ def _collect_clang_module_inputs(
             direct_inputs.append(module_map)
 
         precompiled_module = clang_module.precompiled_module
+        use_precompiled_module = (
+            prefer_precompiled_modules and precompiled_module
+        )
 
-        if prefer_precompiled_modules and precompiled_module:
+        if use_precompiled_module:
             # For builds preferring explicit modules, use it if we have it
             # and don't include any headers as inputs.
             direct_inputs.append(precompiled_module)
-        else:
+
+        if not use_precompiled_module or always_include_headers:
             # If we don't have an explicit module (or we're not using it), we
             # need the transitive headers from the compilation context
             # associated with the module. This will likely overestimate the
@@ -1520,6 +1528,11 @@ def _dependencies_clang_modulemaps_configurator(prerequisites, args):
         compilation_context = prerequisites.cc_compilation_context
 
     return _collect_clang_module_inputs(
+        always_include_headers = getattr(
+            prerequisites,
+            "always_include_headers",
+            False,
+        ),
         explicit_module_compilation_context = compilation_context,
         modules = modules,
         prefer_precompiled_modules = False,
@@ -1550,6 +1563,11 @@ def _dependencies_clang_modules_configurator(prerequisites, args):
         compilation_context = prerequisites.cc_compilation_context
 
     return _collect_clang_module_inputs(
+        always_include_headers = getattr(
+            prerequisites,
+            "always_include_headers",
+            False,
+        ),
         explicit_module_compilation_context = compilation_context,
         modules = modules,
         prefer_precompiled_modules = True,