builddb: support builds with empty configuration

Rotates the UUID to indicate changed behavior.

See: https://discourse.cmake.org/t/15654

Help/dev/experimental.rst

@@ -87,7 +87,7 @@ Build database support
 In order to activate support for exporting build databases, set
 
 * variable ``CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE`` to
-* value ``73194a1d-c0b5-41b9-9190-a4512925e192``.
+* value ``70ef007e-b743-492d-9407-e35eeac03a40``.
 
 This UUID may change in future versions of CMake.  Be sure to use the value
 documented here by the source tree of the version of CMake with which you are

Source/cmExperimental.cxx

@@ -48,7 +48,7 @@ cmExperimental::FeatureData const LookupTable[] = {
     cmExperimental::TryCompileCondition::Always },
   // ExportBuildDatabase
   { "ExportBuildDatabase",
-    "73194a1d-c0b5-41b9-9190-a4512925e192",
+    "70ef007e-b743-492d-9407-e35eeac03a40",
     "CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE",
     "CMake's support for exporting build databases is experimental. It is "
     "meant only for experimentation and feedback to CMake developers.",

Source/cmGeneratorTarget.cxx

@@ -141,7 +141,7 @@ cmGeneratorTarget::cmGeneratorTarget(cmTarget* t, cmLocalGenerator* lg)
   }
 
   auto configs =
-    this->Makefile->GetGeneratorConfigs(cmMakefile::ExcludeEmptyConfig);
+    this->Makefile->GetGeneratorConfigs(cmMakefile::IncludeEmptyConfig);
   std::string build_db_languages[] = { "CXX" };
   for (auto const& language : build_db_languages) {
     for (auto const& config : configs) {

Source/cmGlobalGenerator.cxx

@@ -3447,6 +3447,31 @@ bool cmGlobalGenerator::AddBuildDatabaseTargets()
 
   std::string module_languages[] = { "CXX" };
 
+  // Handle config-less builds.
+  if (configs.empty()) {
+    std::vector<std::string> all_lang_paths;
+    for (auto const& lang : module_languages) {
+      auto comment = cmStrCat("Combining module command databases for ", lang);
+      auto output = cmStrCat(mf->GetHomeOutputDirectory(), "/build_database_",
+                             lang, ".json");
+      mf->GetOrCreateGeneratedSource(output);
+      AddMergeTarget(
+        cmStrCat(TargetPrefix, '-', lang), comment.c_str(), output,
+        [this, lang]() { return this->PerLanguageModuleDbs[lang]; });
+      all_lang_paths.emplace_back(std::move(output));
+    }
+
+    // Add the overall target.
+    auto const* comment = "Combining module command databases";
+    auto output =
+      cmStrCat(mf->GetHomeOutputDirectory(), "/build_database.json");
+    mf->GetOrCreateGeneratedSource(output);
+    AddMergeTarget(std::string{ TargetPrefix }, comment, output,
+                   [all_lang_paths]() { return all_lang_paths; });
+
+    return true;
+  }
+
   // Add per-configuration targets.
   for (auto const& config : configs) {
     // Add per-language targets.

Tests/RunCMake/CXXModules/NinjaDependInfoCompileDatabase.cmake

@@ -4,7 +4,7 @@ set(CMAKE_INTERMEDIATE_DIR_STRATEGY FULL CACHE STRING "" FORCE)
 # here.
 set(CMAKE_CXX_SCANDEP_SOURCE "")
 
-set(CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE "73194a1d-c0b5-41b9-9190-a4512925e192")
+set(CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE "70ef007e-b743-492d-9407-e35eeac03a40")
 
 enable_language(CXX)
 

Tests/RunCMake/CXXModulesCompile/export-build-database-setup.cmake

@@ -1,4 +1,4 @@
-set(CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE "73194a1d-c0b5-41b9-9190-a4512925e192")
+set(CMAKE_EXPERIMENTAL_EXPORT_BUILD_DATABASE "70ef007e-b743-492d-9407-e35eeac03a40")
 
 # _MBCS default append for MSVC ABI workaround: see CMP0204
 # So, force add the default value for all platforms to be consistent.