cmTarget: add support for C++ module fileset types

C++ modules have two variants which are of importance to CMake:

  - `CXX_MODULES`: interface modules (those using `export module M;`,
    `export module M:part;`, or `module M:internal_part;`)
  - `CXX_MODULE_HEADER_UNITS`: importable header units

Creating C++ modules or partitions are *not* supported in any other
source listing. This is because the source files must be installed (so
their scope matters), but not part of usage requirements (what it means
for a module source to be injected into a consumer is not clear at this
moment). Due to the way `FILE_SET` works with scopes, they are a perfect
fit as long as `INTERFACE` is not allowed (which it is not).

1 files changed, 89 insertions, 1 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 8a7215b..6fb28ae 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -1707,7 +1707,8 @@ void addFileSetEntry(cmGeneratorTarget const* headTarget,
         }
       }
       if (!found) {
-        if (fileSet->GetType() == "HEADERS"_s) {
+        if (fileSet->GetType() == "HEADERS"_s ||
+            fileSet->GetType() == "CXX_MODULE_HEADER_UNITS"_s) {
           headTarget->Makefile->GetOrCreateSourceGroup("Header Files")
             ->AddGroupFile(path);
         }
@@ -1728,6 +1729,20 @@ void AddFileSetEntries(cmGeneratorTarget const* headTarget,
       addFileSetEntry(headTarget, config, dagChecker, headerSet, entries);
     }
   }
+  for (auto const& entry : headTarget->Target->GetCxxModuleSetsEntries()) {
+    for (auto const& name : cmExpandedList(entry.Value)) {
+      auto const* cxxModuleSet = headTarget->Target->GetFileSet(name);
+      addFileSetEntry(headTarget, config, dagChecker, cxxModuleSet, entries);
+    }
+  }
+  for (auto const& entry :
+       headTarget->Target->GetCxxModuleHeaderSetsEntries()) {
+    for (auto const& name : cmExpandedList(entry.Value)) {
+      auto const* cxxModuleHeaderSet = headTarget->Target->GetFileSet(name);
+      addFileSetEntry(headTarget, config, dagChecker, cxxModuleHeaderSet,
+                      entries);
+    }
+  }
 }
 
 bool processSources(cmGeneratorTarget const* tgt,
@@ -8700,3 +8715,76 @@ std::string cmGeneratorTarget::GenerateHeaderSetVerificationFile(
 
   return filename;
 }
+
+bool cmGeneratorTarget::HaveCxx20ModuleSources() const
+{
+  auto const& fs_names = this->Target->GetAllFileSetNames();
+  return std::any_of(fs_names.begin(), fs_names.end(),
+                     [this](std::string const& name) -> bool {
+                       auto const* file_set = this->Target->GetFileSet(name);
+                       if (!file_set) {
+                         this->Makefile->IssueMessage(
+                           MessageType::INTERNAL_ERROR,
+                           cmStrCat("Target \"", this->Target->GetName(),
+                                    "\" is tracked to have file set \"", name,
+                                    "\", but it was not found."));
+                         return false;
+                       }
+
+                       auto const& fs_type = file_set->GetType();
+                       return fs_type == "CXX_MODULES"_s ||
+                         fs_type == "CXX_MODULE_HEADER_UNITS"_s;
+                     });
+}
+
+cmGeneratorTarget::Cxx20SupportLevel cmGeneratorTarget::HaveCxxModuleSupport(
+  std::string const& config) const
+{
+  auto const* state = this->Makefile->GetState();
+  if (!state->GetLanguageEnabled("CXX")) {
+    return Cxx20SupportLevel::MissingCxx;
+  }
+  cmStandardLevelResolver standardResolver(this->Makefile);
+  if (!standardResolver.HaveStandardAvailable(this, "CXX", config,
+                                              "cxx_std_20")) {
+    return Cxx20SupportLevel::NoCxx20;
+  }
+  if (!this->Makefile->IsOn("CMAKE_EXPERIMENTAL_CXX_MODULE_DYNDEP")) {
+    return Cxx20SupportLevel::MissingExperimentalFlag;
+  }
+  return Cxx20SupportLevel::Supported;
+}
+
+void cmGeneratorTarget::CheckCxxModuleStatus(std::string const& config) const
+{
+  // Check for `CXX_MODULE*` file sets and a lack of support.
+  if (this->HaveCxx20ModuleSources()) {
+    switch (this->HaveCxxModuleSupport(config)) {
+      case cmGeneratorTarget::Cxx20SupportLevel::MissingCxx:
+        this->Makefile->IssueMessage(
+          MessageType::FATAL_ERROR,
+          cmStrCat("The \"", this->GetName(),
+                   "\" target has C++ module sources but the \"CXX\" language "
+                   "has not been enabled"));
+        break;
+      case cmGeneratorTarget::Cxx20SupportLevel::MissingExperimentalFlag:
+        this->Makefile->IssueMessage(
+          MessageType::FATAL_ERROR,
+          cmStrCat("The \"", this->GetName(),
+                   "\" target has C++ module sources but its experimental "
+                   "support has not been requested"));
+        break;
+      case cmGeneratorTarget::Cxx20SupportLevel::NoCxx20:
+        this->Makefile->IssueMessage(
+          MessageType::FATAL_ERROR,
+          cmStrCat(
+            "The \"", this->GetName(),
+            "\" target has C++ module sources but is not using at least "
+            "\"cxx_std_20\""));
+        break;
+      case cmGeneratorTarget::Cxx20SupportLevel::Supported:
+        // All is well.
+        break;
+    }
+  }
+}