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

1 files changed, 16 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()) {