[build] Make the new --cross-compile-build-swift-tools flag public

This new flag makes it easy to build Swift cross-compilation toolchains, by
disabling cross-compilation of all host tools, like the Swift compiler and
various macros, building on prior pulls #38441 and #82163.

Author: finagolfin

utils/build_swift/build_swift/driver_arguments.py

@@ -689,6 +689,12 @@ def create_argument_parser():
                 "for each cross-compiled toolchain's destdir, useful when building "
                 "multiple toolchains and can be disabled if only cross-compiling one.")
 
+    option('--cross-compile-build-swift-tools', toggle_true,
+           default=True,
+           help="Cross-compile the Swift compiler, other host tools from the "
+                "compiler repository, and various macros for each listed "
+                "--cross-compile-hosts platform.")
+
     option('--stdlib-deployment-targets', store,
            type=argparse.ShellSplitType(),
            default=None,

utils/build_swift/tests/expected_options.py

@@ -156,6 +156,7 @@
     'compiler_vendor': defaults.COMPILER_VENDOR,
     'coverage_db': None,
     'cross_compile_append_host_target_to_destdir': True,
+    'cross_compile_build_swift_tools': True,
     'cross_compile_deps_path': None,
     'cross_compile_hosts': [],
     'infer_cross_compile_hosts_on_darwin': False,
@@ -628,6 +629,7 @@ class BuildScriptImplOption(_BaseOption):
     EnableOption('--build-swift-clang-overlays'),
     EnableOption('--build-swift-remote-mirror'),
     EnableOption('--cross-compile-append-host-target-to-destdir'),
+    EnableOption('--cross-compile-build-swift-tools'),
     EnableOption('--color-in-tests'),
     EnableOption('--distcc'),
     EnableOption('--sccache'),

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -119,6 +119,8 @@ def convert_to_impl_arguments(self):
             "--cmake-generator", args.cmake_generator,
             "--cross-compile-append-host-target-to-destdir", str(
                 args.cross_compile_append_host_target_to_destdir).lower(),
+            "--cross-compile-build-swift-tools", str(
+                args.cross_compile_build_swift_tools).lower(),
             "--build-jobs", str(args.build_jobs),
             "--lit-jobs", str(args.lit_jobs),
             "--common-cmake-options=%s" % ' '.join(

utils/swift_build_support/swift_build_support/products/cmake_product.py

@@ -25,7 +25,7 @@ def is_verbose(self):
 
     def build_with_cmake(self, build_targets, build_type, build_args,
                          prefer_native_toolchain=False, 
-                         ignore_extra_cmake_options=False):
+                         ignore_extra_cmake_options=False, build_llvm=True):
         assert self.toolchain.cmake is not None
         cmake_build = []
         _cmake = cmake.CMake(self.args, self.toolchain,
@@ -73,9 +73,7 @@ def build_with_cmake(self, build_targets, build_type, build_args,
                            env=env)
 
         is_llvm = self.product_name() == "llvm"
-        if (not is_llvm and not self.args.skip_build) or (
-            is_llvm and self.args._build_llvm
-        ):
+        if (not is_llvm and not self.args.skip_build) or (is_llvm and build_llvm):
             cmake_opts = [self.build_dir, "--config", build_type]
 
             shell.call(

utils/swift_build_support/swift_build_support/products/llvm.py

@@ -251,10 +251,10 @@ def build(self, host_target):
                 # space/time efficient than -g on that platform.
                 llvm_cmake_options.define('LLVM_USE_SPLIT_DWARF:BOOL', 'YES')
 
-        if not self.args._build_llvm:
-            # Indicating we don't want to build LLVM at all should
-            # override everything.
-            build_targets = []
+        build = True
+        if not self.args._build_llvm or (not self.args.cross_compile_build_swift_tools
+                                         and self.is_cross_compile_target(host_target)):
+            build = False
         elif self.args.skip_build or not self.args.build_llvm:
             # We can't skip the build completely because the standalone
             # build of Swift depends on these.
@@ -456,7 +456,8 @@ def build(self, host_target):
 
         self._handle_cxx_headers(host_target, platform)
 
-        self.build_with_cmake(build_targets, self.args.llvm_build_variant, [])
+        self.build_with_cmake(build_targets, self.args.llvm_build_variant, [],
+                              build_llvm=build)
 
         # copy over the compiler-rt builtins for iOS/tvOS/watchOS to ensure
         # that Swift's stdlib can use compiler-rt builtins when targeting
@@ -541,7 +542,9 @@ def should_install(self, host_target):
         Whether or not this product should be installed with the given
         arguments.
         """
-        return self.args.install_llvm
+        return self.args.install_llvm and (
+            self.args.cross_compile_build_swift_tools or
+            not self.is_cross_compile_target(host_target))
 
     def install(self, host_target):
         """

utils/swift_build_support/swift_build_support/products/swift_testing_macros.py

@@ -42,13 +42,24 @@ def should_clean(self, host_target):
         return True
 
     def should_build(self, host_target):
-        return True
+        build_macros = not self.is_cross_compile_target(host_target) or \
+            self.args.cross_compile_build_swift_tools
+        if not build_macros:
+            print("Skipping building Testing Macros for %s, because the host tools "
+                  "are not being built" % host_target)
+        return build_macros
 
     def should_test(self, host_target):
         return False
 
     def should_install(self, host_target):
-        return self.args.install_swift_testing_macros
+        install_macros = self.args.install_swift_testing_macros and \
+            (not self.is_cross_compile_target(host_target) or
+             self.args.cross_compile_build_swift_tools)
+        if self.args.install_swift_testing_macros and not install_macros:
+            print("Skipping installing Testing Macros for %s, because the host tools "
+                  "are not being built" % host_target)
+        return install_macros
 
     def _cmake_product(self, host_target):
         build_root = os.path.dirname(self.build_dir)