[clang] [clang] Constant-evaluate format strings as last resort (PR #135864)

[via cfe-commits]

================
@@ -791,10 +791,26 @@ class Expr : public ValueStmt {
                                  const Expr *PtrExpression, ASTContext &Ctx,
                                  EvalResult &Status) const;
 
-  /// If the current Expr can be evaluated to a pointer to a null-terminated
-  /// constant string, return the constant string (without the terminating
-  /// null).
-  std::optional<std::string> tryEvaluateString(ASTContext &Ctx) const;
+  class StringEvalResult {
+    std::string Storage;
+    const StringLiteral *SL;
+    uint64_t Offset;
+
+  public:
+    StringEvalResult(std::string Contents);
+    StringEvalResult(const StringLiteral *SL, uint64_t Offset);
+
+    llvm::StringRef getString() const;
+    bool getStringLiteral(const StringLiteral *&SL, uint64_t &Offset) const;
+  };
----------------
apple-fcloutier wrote:

This is a polite `union` of `std::string` and `StringLiteral`: `Storage` is empty when `SL` is set, and vice versa. `getString()` encapsulates useful logic to decide how to get a `StringRef` back, which I think justifies having a separate type instead of an aggregate. I will add a comment on top of it.

https://github.com/llvm/llvm-project/pull/135864