[clang] [C] Diagnose use of C++ keywords in C (PR #137234)

[Aaron Ballman via cfe-commits]

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@@ -6107,6 +6109,29 @@ static bool isFromSystemHeader(SourceManager &SM, const Decl *D) {
          SM.isInSystemMacro(D->getLocation());
 }
 
+static bool isKeywordInCPlusPlus(const Sema &S, const IdentifierInfo *II) {
+  if (!II)
+    return false;
+
+  // Build a static map of identifiers for all of the keywords in C++ that are
+  // not keywords in C. This allows us to do pointer comparisons instead of
+  // string comparisons when deciding whether the given identifier is a keyword
+  // or not. Note, this treats all keywords as being enabled, regardless of the
+  // setting of other language options. It intentionally disables the modules
+  // keywords because those are conditional keywords, so may be safe to use.
+  static llvm::SmallPtrSet<IdentifierInfo *, 32> Keywords;
+  if (Keywords.empty()) {
+#define MODULES_KEYWORD(NAME)
+#define KEYWORD(NAME, FLAGS)                                                   \
+  Keywords.insert(&S.getPreprocessor().getIdentifierTable().get(#NAME));
+#define CXX_KEYWORD_OPERATOR(NAME, TOK)                                        \
+  Keywords.insert(&S.getPreprocessor().getIdentifierTable().get(#NAME));
+#include "clang/Basic/TokenKinds.def"
+  }
+
+  return Keywords.contains(II);
----------------
AaronBallman wrote:

I'd really rather not try to muck about with the layout of things in the identifier table, that's actually some pretty complicated logic because of "interesting identifiers" and Objective-C. I think the current perf is reasonable given that the checking only happens if the (off by default) diagnostic is enabled. WDYT?

https://github.com/llvm/llvm-project/pull/137234