CXXModules: add a variable for BMI-only compilation

This will be required when dealing with imported targets which contain
modules.

.gitlab/ci/cxx_modules_rules_gcc.cmake

@@ -17,3 +17,4 @@ string(CONCAT CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG
   " -fdeps-format=p1689r5"
   # Force C++ as a language.
   " -x c++")
+set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG "-fmodule-only")

Help/dev/experimental.rst

@@ -99,6 +99,10 @@ dependencies to the file specified by the ``<DYNDEP_FILE>`` placeholder. The
 ``CMAKE_EXPERIMENTAL_CXX_SCANDEP_DEPFILE_FORMAT`` file may be set to ``msvc``
 for scandep rules which use ``msvc``-style dependency reporting.
 
+In order to support ``IMPORTED`` targets with associated C++20 module sources,
+the ``CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG`` variable must be provided
+to have the compiler only output a BMI instead of a BMI and an object file.
+
 The module dependencies should be written in the format described
 by the `P1689r5`_ paper.
 
@@ -113,6 +117,8 @@ For compilers that generate module maps, tell CMake as follows:
   set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "gcc")
   set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG
     "${compiler_flags_for_module_map} -fmodule-mapper=<MODULE_MAP_FILE>")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG
+    "-fmodule-only")
 
 Currently, the only supported formats are, ``clang``, ``gcc``, and ``msvc``.
 The ``gcc`` format is described in the GCC documentation, but the relevant

Modules/Compiler/Clang-CXX.cmake

@@ -43,5 +43,6 @@ if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 16.0)
       " > <DYNDEP_FILE>")
     set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "clang")
     set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
+    set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG "--precompile")
   endif()
 endif()

Modules/Compiler/MSVC-CXX.cmake

@@ -87,4 +87,5 @@ if (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL "19.34")
   set(CMAKE_EXPERIMENTAL_CXX_SCANDEP_DEPFILE_FORMAT "msvc")
   set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FORMAT "msvc")
   set(CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG "@<MODULE_MAP_FILE>")
+  set(CMAKE_EXPERIMENTAL_CXX_MODULE_BMI_ONLY_FLAG "-ifcOnly;-ifcOutput;<OBJECT>")
 endif ()