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".

8 files changed, 125 insertions, 0 deletions

diff --git a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
index 921fabd..81a086a 100644
--- a/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
+++ b/Tests/RunCMake/CXXModules/RunCMakeTest.cmake
@@ -138,6 +138,7 @@ if ("named" IN_LIST CMake_TEST_MODULE_COMPILATION)
   set(RunCMake_CXXModules_NO_TEST 1)
   run_cxx_module_test(circular)
   unset(RunCMake_CXXModules_NO_TEST)
+  run_cxx_module_test(scan_properties)
 endif ()
 
 # Tests which use named modules in shared libraries.
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties-stderr.txt b/Tests/RunCMake/CXXModules/examples/scan_properties-stderr.txt
new file mode 100644
index 0000000..7d79bad
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties-stderr.txt
@@ -0,0 +1,9 @@
+CMake Warning \(dev\) at CMakeLists.txt:20 \(target_sources\):
+  CMake's C\+\+ module support is experimental.  It is meant only for
+  experimentation and feedback to CMake developers.
+This warning is for project developers.  Use -Wno-dev to suppress it.
+
+CMake Warning \(dev\):
+  C\+\+20 modules support via CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP is
+  experimental.  It is meant only for compiler developers to try.
+This warning is for project developers.  Use -Wno-dev to suppress it.
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/CMakeLists.txt b/Tests/RunCMake/CXXModules/examples/scan_properties/CMakeLists.txt
new file mode 100644
index 0000000..551c55c
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/CMakeLists.txt
@@ -0,0 +1,54 @@
+cmake_minimum_required(VERSION 3.24)
+project(scan_properties CXX)
+
+include("${CMAKE_SOURCE_DIR}/../cxx-modules-rules.cmake")
+
+# To detect that not-to-be scanned sources are not scanned, add a `-D` to the
+# scan flags so that the files can detect whether scanning happened and error
+# if not.
+string(APPEND CMAKE_EXPERIMENTAL_CXX_MODULE_MAP_FLAG
+  " -DCMAKE_SCANNED_THIS_SOURCE")
+string(APPEND CMAKE_EXPERIMENTAL_CXX_SCANDEP_SOURCE
+  " -DCMAKE_SCANNED_THIS_SOURCE")
+
+set_property(SOURCE always_scan.cxx
+  PROPERTY CXX_SCAN_FOR_MODULES 1)
+set_property(SOURCE never_scan.cxx
+  PROPERTY CXX_SCAN_FOR_MODULES 0)
+
+add_executable(scans_everything)
+target_sources(scans_everything
+  PRIVATE
+    main.cxx
+    join.cxx
+    always_scan.cxx
+    never_scan.cxx
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        module.cxx)
+target_compile_features(scans_everything PRIVATE cxx_std_20)
+target_compile_definitions(scans_everything PRIVATE SCAN_AT_TARGET_LEVEL=1)
+
+set(CMAKE_CXX_SCAN_FOR_MODULES 0)
+
+add_executable(no_scan_everything)
+target_sources(no_scan_everything
+  PRIVATE
+    main.cxx
+    join.cxx
+    always_scan.cxx
+    never_scan.cxx
+  PRIVATE
+    FILE_SET CXX_MODULES
+      BASE_DIRS
+        "${CMAKE_CURRENT_SOURCE_DIR}"
+      FILES
+        module.cxx)
+target_compile_features(no_scan_everything PRIVATE cxx_std_20)
+target_compile_definitions(no_scan_everything PRIVATE SCAN_AT_TARGET_LEVEL=0)
+
+add_test(NAME scanned COMMAND scans_everything)
+add_test(NAME unscanned COMMAND no_scan_everything)
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/always_scan.cxx b/Tests/RunCMake/CXXModules/examples/scan_properties/always_scan.cxx
new file mode 100644
index 0000000..27087d7
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/always_scan.cxx
@@ -0,0 +1,10 @@
+#ifndef CMAKE_SCANNED_THIS_SOURCE
+#  error "This file should have been scanned"
+#endif
+
+import M;
+
+int scanned_file()
+{
+  return from_module();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/join.cxx b/Tests/RunCMake/CXXModules/examples/scan_properties/join.cxx
new file mode 100644
index 0000000..8184a40
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/join.cxx
@@ -0,0 +1,17 @@
+#if SCAN_AT_TARGET_LEVEL
+#  ifndef CMAKE_SCANNED_THIS_SOURCE
+#    error "This file should have been scanned"
+#  endif
+#else
+#  ifdef CMAKE_SCANNED_THIS_SOURCE
+#    error "This file should not have been scanned"
+#  endif
+#endif
+
+int scanned_file();
+int never_scan();
+
+int join()
+{
+  return scanned_file() + never_scan();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/main.cxx b/Tests/RunCMake/CXXModules/examples/scan_properties/main.cxx
new file mode 100644
index 0000000..81e93f4
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/main.cxx
@@ -0,0 +1,16 @@
+#if SCAN_AT_TARGET_LEVEL
+#  ifndef CMAKE_SCANNED_THIS_SOURCE
+#    error "This file should have been scanned"
+#  endif
+#else
+#  ifdef CMAKE_SCANNED_THIS_SOURCE
+#    error "This file should not have been scanned"
+#  endif
+#endif
+
+int join();
+
+int main(int argc, char** argv)
+{
+  return join();
+}
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/module.cxx b/Tests/RunCMake/CXXModules/examples/scan_properties/module.cxx
new file mode 100644
index 0000000..ad1e04d
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/module.cxx
@@ -0,0 +1,10 @@
+#ifndef CMAKE_SCANNED_THIS_SOURCE
+#  error "This file should have been scanned"
+#endif
+
+export module M;
+
+export int from_module()
+{
+  return 0;
+}
diff --git a/Tests/RunCMake/CXXModules/examples/scan_properties/never_scan.cxx b/Tests/RunCMake/CXXModules/examples/scan_properties/never_scan.cxx
new file mode 100644
index 0000000..8374110
--- /dev/null
+++ b/Tests/RunCMake/CXXModules/examples/scan_properties/never_scan.cxx
@@ -0,0 +1,8 @@
+#ifdef CMAKE_SCANNED_THIS_SOURCE
+#  error "This file should not have been scanned"
+#endif
+
+int never_scan()
+{
+  return 0;
+}