cmGeneratorExpressionNode: implement `COMPILE_ONLY` genex

This generator expression is the inverse of `LINK_ONLY` and only coveys
usage requirements for the purposes of compilation. Its intended use is
to avoid needing to export targets that do not have link usage
requirements (e.g., header-only libraries) when used by another target.

It will also be used to represent private usage requirements on exported
C++ module-containing targets in the future.

Eventually there should be logic to collapse nesting of
`$<COMPILE_ONLY>` and `$<LINK_ONLY>` when generating instances of
either. A TODO is left in the code for this case.

See: #15415

3 files changed, 42 insertions, 0 deletions

diff --git a/Source/cmExportFileGenerator.cxx b/Source/cmExportFileGenerator.cxx
index 6e7ef4e..e997158 100644
--- a/Source/cmExportFileGenerator.cxx
+++ b/Source/cmExportFileGenerator.cxx
@@ -734,6 +734,22 @@ void cmExportFileGenerator::ResolveTargetsInGeneratorExpression(
     lastPos = nameStartPos + libName.size() + 1;
   }
 
+  while (errorString.empty() &&
+         (pos = input.find("$<COMPILE_ONLY:", lastPos)) != std::string::npos) {
+    std::string::size_type nameStartPos = pos + cmStrLen("$<COMPILE_ONLY:");
+    std::string::size_type endPos = input.find('>', nameStartPos);
+    if (endPos == std::string::npos) {
+      errorString = "$<COMPILE_ONLY:...> expression incomplete";
+      break;
+    }
+    std::string libName = input.substr(nameStartPos, endPos - nameStartPos);
+    if (cmGeneratorExpression::IsValidTargetName(libName) &&
+        this->AddTargetNamespace(libName, target, lg)) {
+      input.replace(nameStartPos, endPos - nameStartPos, libName);
+    }
+    lastPos = nameStartPos + libName.size() + 1;
+  }
+
   this->ReplaceInstallPrefix(input);
 
   if (!errorString.empty()) {
diff --git a/Source/cmGeneratorExpressionNode.cxx b/Source/cmGeneratorExpressionNode.cxx
index a47366b..a221498 100644
--- a/Source/cmGeneratorExpressionNode.cxx
+++ b/Source/cmGeneratorExpressionNode.cxx
@@ -1351,6 +1351,29 @@ static const VersionNode<cmSystemTools::OP_LESS> versionLessNode;
 static const VersionNode<cmSystemTools::OP_LESS_EQUAL> versionLessEqNode;
 static const VersionNode<cmSystemTools::OP_EQUAL> versionEqualNode;
 
+static const struct CompileOnlyNode : public cmGeneratorExpressionNode
+{
+  CompileOnlyNode() {} // NOLINT(modernize-use-equals-default)
+
+  std::string Evaluate(
+    const std::vector<std::string>& parameters,
+    cmGeneratorExpressionContext* context,
+    const GeneratorExpressionContent* content,
+    cmGeneratorExpressionDAGChecker* dagChecker) const override
+  {
+    if (!dagChecker) {
+      reportError(context, content->GetOriginalExpression(),
+                  "$<COMPILE_ONLY:...> may only be used via linking");
+      return std::string();
+    }
+    // Linking checks for the inverse, so compiling is the opposite.
+    if (dagChecker->GetTransitivePropertiesOnly()) {
+      return parameters.front();
+    }
+    return std::string();
+  }
+} compileOnlyNode;
+
 static const struct LinkOnlyNode : public cmGeneratorExpressionNode
 {
   LinkOnlyNode() {} // NOLINT(modernize-use-equals-default)
@@ -1366,6 +1389,7 @@ static const struct LinkOnlyNode : public cmGeneratorExpressionNode
                   "$<LINK_ONLY:...> may only be used for linking");
       return std::string();
     }
+    // Compile-only checks for the inverse, so linking is the opposite.
     if (!dagChecker->GetTransitivePropertiesOnly()) {
       return parameters.front();
     }
@@ -3805,6 +3829,7 @@ const cmGeneratorExpressionNode* cmGeneratorExpressionNode::GetNode(
     { "BUILD_LOCAL_INTERFACE", &buildLocalInterfaceNode },
     { "INSTALL_PREFIX", &installPrefixNode },
     { "JOIN", &joinNode },
+    { "COMPILE_ONLY", &compileOnlyNode },
     { "LINK_ONLY", &linkOnlyNode },
     { "COMPILE_LANG_AND_ID", &languageAndIdNode },
     { "COMPILE_LANGUAGE", &languageNode },
diff --git a/Source/cmTargetLinkLibrariesCommand.cxx b/Source/cmTargetLinkLibrariesCommand.cxx
index 0b123b2..03d7c9f 100644
--- a/Source/cmTargetLinkLibrariesCommand.cxx
+++ b/Source/cmTargetLinkLibrariesCommand.cxx
@@ -552,6 +552,7 @@ bool TLL::HandleLibrary(ProcessingState currentProcessingState,
       currentProcessingState == ProcessingPlainPrivateInterface) {
     if (this->Target->GetType() == cmStateEnums::STATIC_LIBRARY ||
         this->Target->GetType() == cmStateEnums::OBJECT_LIBRARY) {
+      // TODO: Detect and no-op `$<COMPILE_ONLY>` genexes here.
       std::string configLib =
         this->Target->GetDebugGeneratorExpressions(lib, llt);
       if (cmGeneratorExpression::IsValidTargetName(lib) ||