cxxmodules: add properties to control scanning

The `CXX_SCAN_FOR_MODULES` property may be used to control scanning for
targets and for source files rather than assuming "C++20 always needs to
be scanned".

1 files changed, 19 insertions, 0 deletions

diff --git a/Help/prop_sf/CXX_SCAN_FOR_MODULES.rst b/Help/prop_sf/CXX_SCAN_FOR_MODULES.rst
new file mode 100644
index 0000000..9d6c0a5
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+++ b/Help/prop_sf/CXX_SCAN_FOR_MODULES.rst
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+CXX_SCAN_FOR_MODULES
+--------------------
+
+.. versionadded:: 3.26
+
+``CXX_SCAN_FOR_MODULES`` is a boolean specifying whether CMake will scan the
+source for C++ module dependencies.  See also the
+:prop_tgt:`CXX_SCAN_FOR_MODULES` for target-wide settings.
+
+When this property is set ``ON``, CMake will scan the source at build time and
+add module dependency information to the compile line as necessary.  When this
+property is set ``OFF``, CMake will not scan the source at build time.  When
+this property is unset, the :prop_tgt:`CXX_SCAN_FOR_MODULES` property is
+consulted.
+
+Note that scanning is only performed if C++20 or higher is enabled for the
+target and the source uses the ``CXX`` language.  Scanning for modules in
+sources belonging to file sets of type ``CXX_MODULES`` and
+``CXX_MODULES_HEADER_UNITS`` is always performed.