cmCxxModuleMapper: track transitive modules for MSVC

MSVC needs the transitive closure of module usage to compile.

Source/cmCxxModuleMapper.h

@@ -5,12 +5,15 @@
 #include "cmConfigure.h" // IWYU pragma: keep
 
 #include <functional>
+#include <map>
+#include <set>
 #include <string>
+#include <vector>
 
 #include <cm/optional>
 #include <cmext/string_view>
 
-struct cmScanDepInfo;
+#include "cmScanDepFormat.h"
 
 enum class CxxModuleMapFormat
 {
@@ -37,10 +40,42 @@ struct CxxModuleLocations
     std::string const& logical_name) const;
 };
 
+struct CxxModuleReference
+{
+  // The path to the module file used.
+  std::string Path;
+  // How the module was looked up.
+  LookupMethod Method;
+};
+
+struct CxxModuleUsage
+{
+  // The usage requirements for this object.
+  std::map<std::string, std::set<std::string>> Usage;
+
+  // The references for this object.
+  std::map<std::string, CxxModuleReference> Reference;
+
+  // Add a reference to a module.
+  //
+  // Returns `true` if it matches how it was found previously, `false` if it
+  // conflicts.
+  bool AddReference(std::string const& logical, std::string const& loc,
+                    LookupMethod method);
+};
+
 // Return the extension to use for a given modulemap format.
 cm::static_string_view CxxModuleMapExtension(
   cm::optional<CxxModuleMapFormat> format);
 
+// Fill in module usage information for internal usages.
+//
+// Returns the set of unresolved module usage requirements (these form an
+// import cycle).
+std::set<std::string> CxxModuleUsageSeed(
+  CxxModuleLocations const& loc, std::vector<cmScanDepInfo> const& objects,
+  CxxModuleUsage& usages);
+
 // Return the contents of the module map in the given format for the
 // object file.
 std::string CxxModuleMapContent(CxxModuleMapFormat format,