FILE_SET: Add CXX_SCAN_FOR_MODULES property for per-file-set scanning control

This is more specific than both the target property and the
directory-wide source file property of the same name.

Help/manual/cmake-cxxmodules.7.rst

@@ -50,8 +50,11 @@ scan or not is used.
   be scanned.
 - If the target does not use at least C++ 20, it will not be scanned.
 - If the source file is not the language ``CXX``, it will not be scanned.
-- If the :prop_sf:`CXX_SCAN_FOR_MODULES` source file property is set, its
+- If the source file belongs to a file set that is not of type ``CXX_MODULES``,
+  and the :prop_fs:`CXX_SCAN_FOR_MODULES` file set property is set, its
   value will be used.
+- If the :prop_sf:`CXX_SCAN_FOR_MODULES` source file property is set in the
+  target's directory, its value will be used.
 - If the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is set, its value
   will be used.  Set the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable
   to initialize this property on all targets as they are created.

Help/manual/cmake-properties.7.rst

@@ -543,6 +543,7 @@ Properties on File Sets
    /prop_fs/BASE_DIRS
    /prop_fs/COMPILE_DEFINITIONS
    /prop_fs/COMPILE_OPTIONS
+   /prop_fs/CXX_SCAN_FOR_MODULES
    /prop_fs/INCLUDE_DIRECTORIES
    /prop_fs/INTERFACE_COMPILE_DEFINITIONS
    /prop_fs/INTERFACE_COMPILE_OPTIONS

Help/prop_fs/CXX_SCAN_FOR_MODULES.rst

@@ -0,0 +1,20 @@
+CXX_SCAN_FOR_MODULES
+--------------------
+
+.. versionadded:: 4.4
+
+``CXX_SCAN_FOR_MODULES`` is a boolean specifying whether CMake will scan the
+file set's sources for C++ module dependencies. See also the
+:prop_sf:`CXX_SCAN_FOR_MODULES` source file property and the
+:prop_tgt:`CXX_SCAN_FOR_MODULES` target property settings.
+
+When this property is set ``ON``, CMake will scan the file set's sources 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 sources
+at build time.  When this property is unset, the
+:prop_sf:`CXX_SCAN_FOR_MODULES` source file property and
+:prop_tgt:`CXX_SCAN_FOR_MODULES` target property are consulted, in that order.
+
+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`` is always performed.

Help/prop_sf/CXX_SCAN_FOR_MODULES.rst

@@ -5,12 +5,14 @@ CXX_SCAN_FOR_MODULES
 
 ``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.
+:prop_fs:`CXX_SCAN_FOR_MODULES` file set property which, if set,
+overrides the per-source settings and the
+:prop_tgt:`CXX_SCAN_FOR_MODULES` target property.
 
 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
+this property is unset, the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is
 consulted.
 
 Note that scanning is only performed if C++20 or higher is enabled for the

Help/prop_tgt/CXX_SCAN_FOR_MODULES.rst

@@ -5,7 +5,8 @@ CXX_SCAN_FOR_MODULES
 
 ``CXX_SCAN_FOR_MODULES`` is a boolean specifying whether CMake will scan C++
 sources in the target for module dependencies.  See also the
-:prop_sf:`CXX_SCAN_FOR_MODULES` for per-source settings which, if set,
+:prop_sf:`CXX_SCAN_FOR_MODULES` for per-source settings and
+:prop_fs:`CXX_SCAN_FOR_MODULES` for file set settings. If either is set, it
 overrides the target-wide settings.
 
 This property is initialized by the value of the

Help/release/dev/FILE_SET-CXX_SCAN_FOR_MODULES.rst

@@ -0,0 +1,5 @@
+FILE_SET-CXX_SCAN_FOR_MODULES
+-----------------------------
+
+* :ref:`File sets <file sets>` learned to support the
+  :prop_fs:`CXX_SCAN_FOR_MODULES` property.

Source/cmGeneratorTarget.cxx

@@ -6050,6 +6050,12 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
   if (targetDyndep == CxxModuleSupport::Unavailable) {
     return false;
   }
+  if (fs) {
+    auto const fsProp = fs->GetProperty("CXX_SCAN_FOR_MODULES");
+    if (fsProp.IsSet()) {
+      return fsProp.IsOn();
+    }
+  }
   auto const sfProp = sf->GetProperty("CXX_SCAN_FOR_MODULES");
   if (sfProp.IsSet()) {
     return sfProp.IsOn();

Tests/RunCMake/CXXModules/NoScanningFileSetProperty-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/CXXModules/NoScanningFileSetProperty-stderr.txt

@@ -0,0 +1,17 @@
+(CMake Error in CMakeLists\.txt:
+  The target named "noscanning-fs-property" has C\+\+ sources that may use
+  modules, but the compiler does not provide a way to discover the import
+  graph dependencies\.  See the cmake-cxxmodules\(7\) manual for details\.  Use
+  the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable scanning\.
+|CMake Error in CMakeLists\.txt:
+  The target named "noscanning-fs-property" has C\+\+ sources that may use
+  modules, but modules are not supported by this generator:
+
+    [^
+]+
+
+  Modules are supported only by Ninja, Ninja Multi-Config, and Visual Studio
+  generators for VS 17\.4 and newer\.  See the cmake-cxxmodules\(7\) manual for
+  details\.  Use the CMAKE_CXX_SCAN_FOR_MODULES variable to enable or disable
+  scanning\.
+)

Tests/RunCMake/CXXModules/NoScanningFileSetProperty.cmake

@@ -0,0 +1,16 @@
+enable_language(CXX)
+unset(CMAKE_CXX_SCANDEP_SOURCE)
+
+add_executable(noscanning-fs-property)
+target_sources(noscanning-fs-property PRIVATE
+  FILE_SET SOURCES FILES sources/module-use.cxx)
+set_target_properties(noscanning-fs-property
+  PROPERTIES
+    CXX_STANDARD 20
+    CXX_STANDARD_REQUIRED ON
+    CXX_SCAN_FOR_MODULES 0)
+set_source_files_properties(sources/module-use.cxx
+  PROPERTIES
+    CXX_SCAN_FOR_MODULES 0)
+set_property(FILE_SET SOURCES TARGET noscanning-fs-property
+             PROPERTY CXX_SCAN_FOR_MODULES 1)

Tests/RunCMake/CXXModules/RunCMakeTest.cmake

@@ -22,6 +22,7 @@ if ("cxx_std_20" IN_LIST CMAKE_CXX_COMPILE_FEATURES)
   endif ()
 
   run_cmake(NoScanningSourceFileProperty)
+  run_cmake(NoScanningFileSetProperty)
   run_cmake(NoScanningTargetProperty)
   run_cmake(NoScanningVariable)
   run_cmake(CMP0155-OLD)