Add swift.internalize_at_link feature and enable it by default

This feature controls the `-Xfrontend -internalize-at-link` flag, which allows the linker to hide public symbols that are not being used and thus dead-strip them. Users of frameworks will need to explicitly export them with an `-exported_symbols_list` if they are not already.

PiperOrigin-RevId: 524870535
(cherry picked from commit e8838f2)
Signed-off-by: Brentley Jones <github@brentleyjones.com>

swift/internal/feature_names.bzl

@@ -333,6 +333,12 @@ SWIFT_FEATURE_ENABLE_BARE_SLASH_REGEX = "swift.supports_bare_slash_regex"
 # Clang SPIs as Swift SPIs.
 SWIFT_FEATURE_DISABLE_CLANG_SPI = "swift.disable_clang_spi"
 
+# If enabled, allow public symbols to be internalized at link time to support
+# better dead-code stripping. This assumes that all clients of public types are
+# part of the same link unit or that public symbols linked into frameworks are
+# explicitly exported via `-exported_symbols_list`.
+SWIFT_FEATURE_INTERNALIZE_AT_LINK = "swift.internalize_at_link"
+
 # Disables Swift sandbox which prevents issues with nested sandboxing when Swift code contains system-provided macros.
 # If enabled '#Preview' macro provided by SwiftUI fails to build and probably other system-provided macros.
 # Enabled by default for Swift 5.10+ on macOS.

swift/toolchains/config/compile_config.bzl

@@ -59,6 +59,7 @@ load(
     "SWIFT_FEATURE_GLOBAL_MODULE_CACHE_USES_TMPDIR",
     "SWIFT_FEATURE_INDEX_INCLUDE_LOCALS",
     "SWIFT_FEATURE_INDEX_WHILE_BUILDING",
+    "SWIFT_FEATURE_INTERNALIZE_AT_LINK",
     "SWIFT_FEATURE_LAYERING_CHECK",
     "SWIFT_FEATURE_MODULAR_INDEXING",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
@@ -365,6 +366,13 @@ def compile_action_configs(
             ],
         ),
 
+        # Improve dead-code stripping.
+        ActionConfigInfo(
+            actions = [SWIFT_ACTION_COMPILE],
+            configurators = [add_arg("-Xfrontend", "-internalize-at-link")],
+            features = [SWIFT_FEATURE_INTERNALIZE_AT_LINK],
+        ),
+
         # Enable or disable serialization of debugging options into
         # swiftmodules.
         ActionConfigInfo(

swift/toolchains/swift_toolchain.bzl

@@ -55,6 +55,7 @@ load(
     "SWIFT_FEATURE_DISABLE_SYSTEM_INDEX",
     "SWIFT_FEATURE_EMIT_SWIFTDOC",
     "SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO",
+    "SWIFT_FEATURE_INTERNALIZE_AT_LINK",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
     "SWIFT_FEATURE_NO_GENERATED_MODULE_MAP",
     "SWIFT_FEATURE_OPT_USES_WMO",
@@ -469,6 +470,7 @@ def _swift_toolchain_impl(ctx):
         SWIFT_FEATURE_DISABLE_SYSTEM_INDEX,
         SWIFT_FEATURE_EMIT_SWIFTDOC,
         SWIFT_FEATURE_EMIT_SWIFTSOURCEINFO,
+        SWIFT_FEATURE_INTERNALIZE_AT_LINK,
         SWIFT_FEATURE_NO_GENERATED_MODULE_MAP,
         SWIFT_FEATURE_OPT_USES_WMO,
         SWIFT_FEATURE_USE_GLOBAL_MODULE_CACHE,

swift/toolchains/xcode_swift_toolchain.bzl

@@ -61,6 +61,7 @@ load(
     "SWIFT_FEATURE_ENABLE_BATCH_MODE",
     "SWIFT_FEATURE_ENABLE_SKIP_FUNCTION_BODIES",
     "SWIFT_FEATURE_FILE_PREFIX_MAP",
+    "SWIFT_FEATURE_INTERNALIZE_AT_LINK",
     "SWIFT_FEATURE_MODULE_MAP_HOME_IS_CWD",
     "SWIFT_FEATURE_OBJC_LINK_FLAGS",
     "SWIFT_FEATURE_OPT_USES_WMO",
@@ -680,6 +681,7 @@ def _xcode_swift_toolchain_impl(ctx):
         SWIFT_FEATURE_ENABLE_BATCH_MODE,
         SWIFT_FEATURE_ENABLE_SKIP_FUNCTION_BODIES,
         SWIFT_FEATURE_FILE_PREFIX_MAP,
+        SWIFT_FEATURE_INTERNALIZE_AT_LINK,
         SWIFT_FEATURE_OBJC_LINK_FLAGS,
         SWIFT_FEATURE_OPT_USES_WMO,
         SWIFT_FEATURE_REMAP_XCODE_PATH,