cxxmodules: generate synthetic targets as an initial pass

We need to be able to construct BMIs that will be usable from the client
modules for the target importing the module, so create BMI-only
compilation rules for `IMPORTED` targets to create these BMIs.

1 files changed, 40 insertions, 0 deletions

diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index 9af6e9b..d5099ee 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -55,6 +55,7 @@
 #include "cmStateDirectory.h"
 #include "cmStateTypes.h"
 #include "cmStringAlgorithms.h"
+#include "cmSyntheticTargetCache.h"
 #include "cmSystemTools.h"
 #include "cmValue.h"
 #include "cmVersion.h"
@@ -1560,6 +1561,17 @@ bool cmGlobalGenerator::Compute()
   }
 #endif
 
+  // Iterate through all targets and set up C++20 module targets.
+  // Create target templates for each imported target with C++20 modules.
+  // INTERFACE library with BMI-generating rules and a collation step?
+  // Maybe INTERFACE libraries with modules files should just do BMI-only?
+  // Make `add_dependencies(imported_target
+  // $<$<TARGET_NAME_IF_EXISTS:uses_imported>:synth1>
+  // $<$<TARGET_NAME_IF_EXISTS:other_uses_imported>:synth2>)`
+  if (!this->DiscoverSyntheticTargets()) {
+    return false;
+  }
+
   // Add generator specific helper commands
   for (const auto& localGen : this->LocalGenerators) {
     localGen->AddHelperCommands();
@@ -1784,6 +1796,34 @@ void cmGlobalGenerator::ComputeTargetOrder(cmGeneratorTarget const* gt,
   entry->second = index++;
 }
 
+bool cmGlobalGenerator::DiscoverSyntheticTargets()
+{
+  cmSyntheticTargetCache cache;
+
+  for (auto const& gen : this->LocalGenerators) {
+    // Because DiscoverSyntheticTargets() adds generator targets, we need to
+    // cache the existing list of generator targets before starting.
+    std::vector<cmGeneratorTarget*> genTargets;
+    genTargets.reserve(gen->GetGeneratorTargets().size());
+    for (auto const& tgt : gen->GetGeneratorTargets()) {
+      genTargets.push_back(tgt.get());
+    }
+
+    for (auto* tgt : genTargets) {
+      std::vector<std::string> const& configs =
+        tgt->Makefile->GetGeneratorConfigs(cmMakefile::IncludeEmptyConfig);
+
+      for (auto const& config : configs) {
+        if (!tgt->DiscoverSyntheticTargets(cache, config)) {
+          return false;
+        }
+      }
+    }
+  }
+
+  return true;
+}
+
 bool cmGlobalGenerator::AddHeaderSetVerification()
 {
   for (auto const& gen : this->LocalGenerators) {