[clang-tools-extra] [clang-tidy] fix false positives when using name-independent variables after C++26 for bugprone-unused-local-non-trivial-variable (PR #121783)

[Piotr Zegar via cfe-commits]

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@@ -29,6 +29,12 @@ static constexpr StringRef DefaultIncludeTypeRegex =
 
 AST_MATCHER(VarDecl, isLocalVarDecl) { return Node.isLocalVarDecl(); }
 AST_MATCHER(VarDecl, isReferenced) { return Node.isReferenced(); }
+AST_MATCHER_P(VarDecl, explicitMarkUnused, LangOptions, LangOpts) {
+  // Implementations should not emit a warning that a name-independent
+  // declaration is used or unused.
+  return Node.hasAttr<UnusedAttr>() ||
+         (LangOpts.CPlusPlus26 && Node.isPlaceholderVar(LangOpts));
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PiotrZSL wrote:

And, you dont need to pass LangOptions via argument.
>From matcher you got access to ASTContext, and that class got [getLangOpts](https://clang.llvm.org/doxygen/classclang_1_1ASTContext.html#a50f31d935973dd09016088579ba81944)

https://github.com/llvm/llvm-project/pull/121783