cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

1 files changed, 16 insertions, 0 deletions

diff --git a/Help/prop_tgt/CXX_MODULE_SETS.rst b/Help/prop_tgt/CXX_MODULE_SETS.rst
new file mode 100644
index 0000000000..c03df391ac
--- /dev/null
+++ b/Help/prop_tgt/CXX_MODULE_SETS.rst
@@ -0,0 +1,16 @@
+CXX_MODULE_SETS
+---------------
+
+.. note ::
+
+  Experimental. Gated by ``CMAKE_EXPERIMENTAL_CXX_MODULE_CMAKE_API``
+
+Read-only list of the target's ``PRIVATE`` and ``PUBLIC`` C++ module sets (i.e.
+all file sets with the type ``CXX_MODULES``). Files listed in these file sets are
+treated as source files for the purpose of IDE integration.
+
+C++ module sets may be defined using the :command:`target_sources` command
+``FILE_SET`` option with type ``CXX_MODULES``.
+
+See also :prop_tgt:`CXX_MODULE_SET_<NAME>`, :prop_tgt:`CXX_MODULE_SET` and
+:prop_tgt:`INTERFACE_CXX_MODULE_SETS`.