[libc++][modules] Re-add build dir CMakeLists.txt.

libcxx/docs/Modules.rst

@@ -218,9 +218,13 @@ Building this project is done with the following steps, assuming the files
 
   $ mkdir build
   $ cmake -G Ninja -S . -B build -DCMAKE_CXX_COMPILER=<path-to-compiler> -DLIBCXX_BUILD=<build>
+  $ ninja -j1 std -C build
   $ ninja -C build
   $ build/main
 
+.. note:: The ``std`` dependencies of ``std.compat`` is not always resolved when
+          building the ``std`` target using multiple jobs.
+
 .. warning:: ``<path-to-compiler>`` should point point to the real binary and
              not to a symlink.
 

libcxx/modules/CMakeLists.txt

@@ -137,6 +137,25 @@ set(LIBCXX_MODULE_STD_COMPAT_SOURCES
   std.compat/cwctype.inc
 )
 
+# TODO MODULES the CMakeLists.txt in the build directory is only temporary.
+# This allows using as available in the build directory. Once build systems
+# have proper support for the installed files this will be removed.
+if ("${LIBCXX_GENERATED_INCLUDE_DIR}" STREQUAL "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}")
+  # This typically happens when the target is not installed.
+  set(LIBCXX_CONFIGURED_INCLUDE_DIRS "${LIBCXX_GENERATED_INCLUDE_DIR}")
+else()
+  # It's important that the arch directory be included first so that its header files
+  # which interpose on the default include dir be included instead of the default ones.
+  set(LIBCXX_CONFIGURED_INCLUDE_DIRS
+    "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR};${LIBCXX_GENERATED_INCLUDE_DIR}"
+  )
+endif()
+configure_file(
+  "CMakeLists.txt.in"
+  "${LIBCXX_GENERATED_MODULE_DIR}/CMakeLists.txt"
+  @ONLY
+)
+
 set(LIBCXX_MODULE_STD_INCLUDE_SOURCES)
 foreach(file ${LIBCXX_MODULE_STD_SOURCES})
   set(
@@ -166,6 +185,7 @@ configure_file(
 )
 
 set(_all_modules)
+list(APPEND _all_modules "${LIBCXX_GENERATED_MODULE_DIR}/CMakeLists.txt")
 list(APPEND _all_modules "${LIBCXX_GENERATED_MODULE_DIR}/std.cppm")
 list(APPEND _all_modules "${LIBCXX_GENERATED_MODULE_DIR}/std.compat.cppm")
 foreach(file ${LIBCXX_MODULE_STD_SOURCES} ${LIBCXX_MODULE_STD_COMPAT_SOURCES})

libcxx/modules/CMakeLists.txt.in

@@ -0,0 +1,88 @@
+cmake_minimum_required(VERSION 3.26)
+
+project(libc++-modules LANGUAGES CXX)
+
+# Enable CMake's module support
+if(CMAKE_VERSION VERSION_LESS "3.28.0")
+  if(CMAKE_VERSION VERSION_LESS "3.27.0")
+    set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "2182bf5c-ef0d-489a-91da-49dbc3090d2a")
+  else()
+    set(CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API "aa1f7df0-828a-4fcd-9afc-2dc80491aca7")
+  endif()
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP 1)
+else()
+  cmake_policy(VERSION 3.28)
+endif()
+
+# Default to C++ extensions being off. Libc++'s modules support have trouble
+# with extensions right now.
+set(CMAKE_CXX_EXTENSIONS OFF)
+
+# Propagates the CMake options to the modules.
+#
+# This uses the std module hard-coded since the std.compat module does not
+# depend on these flags.
+macro(compile_define_if_not condition def)
+  if (NOT ${condition})
+    target_compile_definitions(std PRIVATE ${def})
+  endif()
+endmacro()
+macro(compile_define_if condition def)
+  if (${condition})
+    target_compile_definitions(std PRIVATE ${def})
+  endif()
+endmacro()
+
+### STD
+
+add_library(std)
+target_sources(std
+  PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES
+    std.cppm
+)
+
+target_include_directories(std SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+
+if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
+  target_compile_options(std PUBLIC -fno-exceptions)
+endif()
+
+target_compile_options(std
+  PUBLIC
+    -nostdinc++
+    -Wno-reserved-module-identifier
+    -Wno-reserved-user-defined-literal
+    @LIBCXX_COMPILE_FLAGS@
+)
+set_target_properties(std
+  PROPERTIES
+    OUTPUT_NAME   "c++std"
+)
+
+### STD.COMPAT
+
+add_library(std.compat)
+target_sources(std.compat
+  PUBLIC FILE_SET cxx_modules TYPE CXX_MODULES FILES
+    std.compat.cppm
+)
+
+target_include_directories(std.compat SYSTEM PRIVATE @LIBCXX_CONFIGURED_INCLUDE_DIRS@)
+
+if (NOT @LIBCXX_ENABLE_EXCEPTIONS@)
+  target_compile_options(std.compat PUBLIC -fno-exceptions)
+endif()
+
+target_compile_options(std.compat
+  PUBLIC
+    -nostdinc++
+    -Wno-reserved-module-identifier
+    -Wno-reserved-user-defined-literal
+	-fmodule-file=std=${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/std.dir/std.pcm
+    @LIBCXX_COMPILE_FLAGS@
+)
+set_target_properties(std.compat
+  PROPERTIES
+    OUTPUT_NAME   "c++std.compat"
+)
+add_dependencies(std.compat std)