[clang-tools-extra] [clang-tidy] warn when `true` is used as a preprocessor keyword in C (PR #128265)

[via cfe-commits]

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@@ -0,0 +1,123 @@
+//===--- TrueMacroCheck.cpp - clang-tidy ----------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "TrueMacroCheck.h"
+#include "clang/Lex/MacroInfo.h"
+#include "clang/Lex/PPCallbacks.h"
+#include "clang/Lex/Preprocessor.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::bugprone {
+namespace {
+
+class MacroCallback : public PPCallbacks {
+  static constexpr const char *TrueMacroSpelling = "true";
+
+public:
+  MacroCallback(TrueMacroCheck *Check, Preprocessor *PP)
+      : Check(Check), PP(PP) {}
+  void MacroDefined(const Token &MacroNameTok,
+                    const MacroDirective *MD) override {
+    if (TrueDefined)
+      return;
+
+    const MacroInfo *MI = MD->getMacroInfo();
+    for (const Token &Tok : MI->tokens()) {
+      if (PP->getSpelling(Tok) == TrueMacroSpelling)
+        emitDiagnostics(Tok.getLocation(),
+                        {Tok.getLocation(), Tok.getEndLoc()});
+    }
+
+    if (PP->getSpelling(MacroNameTok) == TrueMacroSpelling)
+      TrueDefined = true;
+  }
+
+  virtual void MacroUndefined(const Token &MacroNameTok,
+                              const MacroDefinition &MD,
+                              const MacroDirective *Undef) override {
+    if (PP->getSpelling(MacroNameTok) == TrueMacroSpelling)
+      TrueDefined = false;
+  }
+
+  virtual void If(SourceLocation Loc, SourceRange ConditionRange,
+                  ConditionValueKind ConditionValue) override {
+    StringRef Condition =
+        Lexer::getSourceText(CharSourceRange::getTokenRange(ConditionRange),
+                             PP->getSourceManager(), PP->getLangOpts());
+
+    if (!TrueDefined && Condition == TrueMacroSpelling) {
+      emitDiagnostics(ConditionRange.getBegin(), ConditionRange);
+      return;
+    }
+
+    for (auto &&Identifier : identifiersInCondition(Condition)) {
+      if (!TrueDefined && Identifier == TrueMacroSpelling) {
+        emitDiagnostics(Loc, {}, true);
+        break;
+      }
+    }
+  }
+
+private:
+  void emitDiagnostics(SourceLocation Loc, SourceRange ReplaceRange,
+                       bool InCondition = false) {
+    DiagnosticBuilder Builder =
+        Check->diag(Loc, "in C 'true'%select{| in the condition}0 is treated "
+                         "as an undefined "
+                         "macro and evaluates to a falsy value; "
+                         "consider replacing it with '1'")
+        << InCondition;
+
+    if (!InCondition)
+      Builder << FixItHint::CreateReplacement(ReplaceRange, "1");
+  }
+
+  std::vector<StringRef> identifiersInCondition(StringRef Condition) {
+    const static auto Start = [](char C) {
+      return C == '_' || ('a' <= C && C <= 'z') || ('A' <= C && C <= 'Z');
+    };
+
+    const static auto Continue = [](char C) {
+      return ('0' <= C && C <= '9') || Start(C);
+    };
+
+    std::vector<StringRef> Identifiers;
+    const char *Str = nullptr;
+
+    for (size_t I = 0; I < Condition.size(); ++I) {
+      char C = Condition[I];
----------------
EugeneZelenko wrote:

```suggestion
      const char C = Condition[I];
```

https://github.com/llvm/llvm-project/pull/128265