[Build Add --minimal-cxx-bootstrap to build a minimal Swift from C++ sources

Introduce an explicit build-script flag `--minimal-cxx-bootstrap` and the
CMake option SWIFT_MINIMAL_CXX_BOOTSTRAP it maps to. This option turns
off components that require a host Swift toolchain, and is intented to
be used when bringing up a new host platform. It disables:

* Macros in the compiler
* Regular expression support in the compiler
* The regular expression support libraries
* Most of the unit-testing facilities for the compiler
* The optimizer passes written in Swift (removing Embedded Swift
support, among other things)

Author: DougGregor

CMakeLists.txt

@@ -816,6 +816,7 @@ option(SWIFT_BUILD_SWIFT_SYNTAX
 option(SWIFT_BUILD_REGEX_PARSER_IN_COMPILER
     "Build the Swift regex parser as part of the compiler."
     TRUE)
+
 if(SWIFT_BUILD_REGEX_PARSER_IN_COMPILER AND NOT SWIFT_BUILD_SWIFT_SYNTAX)
   message(WARNING "Force setting SWIFT_BUILD_REGEX_PARSER_IN_COMPILER=OFF because Swift parser integration is disabled")
   set(SWIFT_BUILD_REGEX_PARSER_IN_COMPILER OFF)
@@ -950,6 +951,26 @@ if (NOT BOOTSTRAPPING_MODE)
   message(FATAL_ERROR "turning off bootstrapping is not supported anymore")
 endif()
 
+option(SWIFT_MINIMAL_CXX_BOOTSTRAP
+    "Build a minimal Swift compiler from only C++ sources. This compiler is only suitable for bootstrapping a Swift toolchain in an environment where one isn't already available."
+    FALSE)
+
+if(SWIFT_MINIMAL_CXX_BOOTSTRAP)
+    message(WARNING "Building minimal Swift compiler from only C++ sources. This compiler is only suitable for bootstrapping a full Swift compiler.")
+
+  # Disable swift-syntax integration
+  set(SWIFT_BUILD_SWIFT_SYNTAX OFF)
+
+  # Disable regular expression support.
+  set(SWIFT_BUILD_REGEX_PARSER_IN_COMPILER OFF)
+
+  # Disable string processing library
+  set(SWIFT_ENABLE_EXPERIMENTAL_STRING_PROCESSING OFF)
+
+  # Disable bootstrapping
+  set(BOOTSTRAPPING_MODE OFF)
+endif()
+
 # As a temporary workaround, disable SwiftCompilerSources on
 # Windows/ARM64 because the compiler segfaults
 if(CMAKE_SYSTEM_NAME STREQUAL "Windows" AND CMAKE_SYSTEM_PROCESSOR MATCHES "ARM64")

utils/build_swift/build_swift/driver_arguments.py

@@ -637,6 +637,12 @@ def create_argument_parser():
            default=defaults.llvm_install_components(),
            help='A semi-colon split list of llvm components to install')
 
+    option('--minimal-cxx-bootstrap', toggle_true, default=False,
+           help='Build a minimal Swift compiler from only C++ sources '
+                '(that does not require a host Swift compiler). The resulting '
+                'toolchain is only suitable for bootstrapping Swift on a new '
+                'host.')
+
     option('--bootstrapping', store('bootstrapping_mode'),
            choices=['hosttools', 'bootstrapping', 'bootstrapping-with-hostlibs'],
            help='The bootstrapping build mode for swift compiler modules. '

utils/build_swift/tests/expected_options.py

@@ -246,6 +246,7 @@
     'lto_type': None,
     'maccatalyst': False,
     'maccatalyst_ios_tests': False,
+    'minimal_cxx_bootstrap': False,
     'musl_path': '/usr/local/musl',
     'linux_static_archs': ['x86_64', 'aarch64'],
     'native_clang_tools_path': None,
@@ -694,6 +695,7 @@ class BuildScriptImplOption(_BaseOption):
     EnableOption('--clean-early-swift-driver', dest='clean_early_swift_driver'),
     EnableOption('--llvm-enable-modules'),
     EnableOption('--build-llvm', dest='_build_llvm'),
+    EnableOption('--minimal-cxx-bootstrap'),
 
     DisableOption('--skip-build-cmark', dest='build_cmark'),
     DisableOption('--skip-build-llvm', dest='build_llvm'),

utils/swift_build_support/swift_build_support/build_script_invocation.py

@@ -275,6 +275,16 @@ def convert_to_impl_arguments(self):
             args.extra_cmake_options.append(
                 '-DSWIFT_EARLY_SWIFT_DRIVER_BUILD={}'.format(swift_driver_build))
 
+        if args.minimal_cxx_bootstrap:
+            if args.build_early_swift_driver:
+                exit_rejecting_arguments(
+                    'cannot combine --minimal-cxx-bootstrap with the early '
+                    'Swift driver. Please remove --minimal-cxx-bootstrap or '
+                    'add --skip-early-swift-driver')
+
+            args.extra_cmake_options.append(
+                '-DSWIFT_MINIMAL_CXX_BOOTSTRAP:BOOL=TRUE')
+
         # Then add subproject install flags that either skip building them /or/
         # if we are going to build them and install_all is set, we also install
         # them.