[libc++][format] Add a CMake Unicode option.

[Mordante]

Summary

This option is used to select between the format headers output column
width option. This option should be independent of the locale setting.
It's encouraged to default to Unicode unless the platform doesn't offer
that option.

[format.string.std]/10

For the purposes of width computation, a string is assumed to be in a
locale-independent, implementation-defined encoding. Implementations
should use a Unicode encoding on platforms capable of displaying Unicode

[Mordante]

Reupload to fix the build breakage. The patch should be stand-alone but seems
to have an issue to apply the patch when not part of the series.

[Mordante]

Rebase.
Fixes minor grammar error in comment.

[vitaut]

LGTM

[Mordante]

Rebase so next patches in the series get the changes done in the previous patches.

[Mordante]

Rebase so other patches in the series use the same base.

[vitaut]

LGTM

[Mordante]

Rebase to get changes in the rest of the patch series.

[ldionne]

libcxx/CMakeLists.txt

122–125

I would not mention the special behavior of <format> specifically, since eventually there will be more that depends on this option.

[Mordante]

Rebase against main.

libcxx/CMakeLists.txt

option(LIBCXX_ENABLE_RANDOM_DEVICE
    supported, most of <random> will still be available, but std::random_device
    will not." ON)
 option(LIBCXX_ENABLE_LOCALIZATION
   "Whether to include support for localization in the library. Disabling
    localization can be useful when porting to platforms that don't support
    the C locale API (e.g. embedded). When localization is not supported,
    several parts of the library will be disabled: <iostream>, <regex>, <locale>
    will be completely unusable, and other parts may be only partly available." ON)
+option(LIBCXX_ENABLE_UNICODE
+  "Whether to include support for Unicode in the library. Disabling Unicode can
+  be useful when porting to platforms that don't support UTF-8 encoding (e.g.
+  embedded). When Unicode is not supported, the <format> header will use a
+  different width estimation algorithm. Instead of using Unicode based
+  estimations it will assume every input character will result in one output
+  character with a width of one column." ON)
 option(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS
   "Whether to turn on vendor availability annotations on declarations that depend
    on definitions in a shared library. By default, we assume that we're not building
    libc++ for any specific vendor, and we disable those annotations. Vendors wishing
    to provide compile-time errors when using features unavailable on some version of
    the shared library they shipped should turn this on and see `include/__availability`
    for more details." OFF)
 option(LIBCXX_ENABLE_INCOMPLETE_FEATURES

 config_define_if(LIBCXX_HAS_WIN32_THREAD_API _LIBCPP_HAS_THREAD_API_WIN32)
 config_define_if(LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY _LIBCPP_HAS_THREAD_LIBRARY_EXTERNAL)
 config_define_if(LIBCXX_HAS_MUSL_LIBC _LIBCPP_HAS_MUSL_LIBC)
 config_define_if(LIBCXX_NO_VCRUNTIME _LIBCPP_NO_VCRUNTIME)
 config_define_if(LIBCXX_ENABLE_PARALLEL_ALGORITHMS _LIBCPP_HAS_PARALLEL_ALGORITHMS)
 config_define_if_not(LIBCXX_ENABLE_FILESYSTEM _LIBCPP_HAS_NO_FILESYSTEM_LIBRARY)
 config_define_if_not(LIBCXX_ENABLE_RANDOM_DEVICE _LIBCPP_HAS_NO_RANDOM_DEVICE)
 config_define_if_not(LIBCXX_ENABLE_LOCALIZATION _LIBCPP_HAS_NO_LOCALIZATION)
+config_define_if_not(LIBCXX_ENABLE_UNICODE _LIBCPP_HAS_NO_UNICODE)
 config_define_if_not(LIBCXX_ENABLE_VENDOR_AVAILABILITY_ANNOTATIONS _LIBCPP_HAS_NO_VENDOR_AVAILABILITY_ANNOTATIONS)
 # Incomplete features get their own specific disabling flags. This makes it
 # easier to grep for target specific flags once the feature is complete.
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_FORMAT)
 config_define_if_not(LIBCXX_ENABLE_INCOMPLETE_FEATURES _LIBCPP_HAS_NO_INCOMPLETE_RANGES)
 
 if (LIBCXX_ABI_DEFINES)
   set(abi_defines)

libcxx/cmake/caches/Generic-no-unicode.cmake

+set(LIBCXX_ENABLE_UNICODE OFF CACHE BOOL "")

libcxx/docs/ReleaseNotes.rst

 
 New Features
 ------------
 
 - There's initial support for the C++20 header ``<format>``. The implementation
   is incomplete. Some functions are known to be inefficient; both in memory
   usage and performance. The implementation is considered experimental and isn't
   considered ABI stable.
+- There's a new CMake option ``LIBCXX_ENABLE_UNICODE`` to disable Unicode
+  support in the ``<format>`` header. This only affects the estimation of the
+  output width of the format functions.
 
 API Changes
 -----------
 
 - ...

libcxx/utils/ci/buildkite-pipeline.yml

- label: "No locale"
     agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:
       automatic:
         - exit_status: -1  # Agent was lost
           limit: 2
 
+  - label: "No Unicode"
+    command: "libcxx/utils/ci/run-buildbot generic-no-unicode"
+    artifact_paths:
+      - "**/test-results.xml"
+    agents:
+      queue: "libcxx-builders"
+    retry:
+      automatic:
+        - exit_status: -1  # Agent was lost
+          limit: 2
+
   - label: "Benchmarks"
     command: "libcxx/utils/ci/run-buildbot benchmarks"
     artifact_paths:
       - "**/test-results.xml"
     agents:
       queue: "libcxx-builders"
       os: "linux"
     retry:

libcxx/utils/ci/run-buildbot

generic-no-random_device)
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-no-random_device.cmake"
     check-cxx-cxxabi
 ;;
 generic-no-localization)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-no-localization.cmake"
     check-cxx-cxxabi
 ;;
+generic-no-unicode)
+    export CC=clang
+    export CXX=clang++
+    clean
+    generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Generic-no-unicode.cmake"
+    check-cxx-cxxabi
+;;
 x86_64-apple-system)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake"
     check-cxx-cxxabi
 ;;
 x86_64-apple-system-noexceptions)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/Apple.cmake" \

libcxx/utils/libcxx/test/features.py

macros = {
   '_LIBCPP_NO_VCRUNTIME': 'libcpp-no-vcruntime',
   '_LIBCPP_ABI_VERSION': 'libcpp-abi-version',
   '_LIBCPP_ABI_UNSTABLE': 'libcpp-abi-unstable',
   '_LIBCPP_HAS_NO_FILESYSTEM_LIBRARY': 'libcpp-has-no-filesystem-library',
   '_LIBCPP_HAS_NO_RANDOM_DEVICE': 'libcpp-has-no-random-device',
   '_LIBCPP_HAS_NO_LOCALIZATION': 'libcpp-has-no-localization',
   '_LIBCPP_HAS_NO_INCOMPLETE_FORMAT': 'libcpp-has-no-incomplete-format',
   '_LIBCPP_HAS_NO_INCOMPLETE_RANGES': 'libcpp-has-no-incomplete-ranges',
+  '_LIBCPP_HAS_NO_UNICODE': 'libcpp-has-no-unicode',
 }
 for macro, feature in macros.items():
   DEFAULT_FEATURES += [
     Feature(name=lambda cfg, m=macro, f=feature: f + (
               '={}'.format(compilerMacros(cfg)[m]) if compilerMacros(cfg)[m] else ''
             ),
             when=lambda cfg, m=macro: m in compilerMacros(cfg),