[clang-tools-extra] [clang-tidy] new check misc-use-internal-linkage (PR #90830)

[Congcong Cai via cfe-commits]

================
@@ -0,0 +1,78 @@
+//===--- UseInternalLinkageCheck.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 "UseInternalLinkageCheck.h"
+#include "../utils/FileExtensionsUtils.h"
+#include "clang/AST/Decl.h"
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/ASTMatchers/ASTMatchers.h"
+#include "clang/ASTMatchers/ASTMatchersMacros.h"
+#include "clang/Basic/SourceLocation.h"
+#include "clang/Basic/Specifiers.h"
+#include "llvm/ADT/STLExtras.h"
+
+using namespace clang::ast_matchers;
+
+namespace clang::tidy::misc {
+
+namespace {
+
+AST_MATCHER(Decl, isFirstDecl) { return Node.isFirstDecl(); }
+
+AST_MATCHER_P(Decl, isInMainFile, FileExtensionsSet, HeaderFileExtensions) {
+  return llvm::all_of(Node.redecls(), [&](const Decl *D) {
+    SourceManager &SM = Finder->getASTContext().getSourceManager();
+    const SourceLocation L = D->getLocation();
+    return SM.isInMainFile(L) &&
+           !utils::isSpellingLocInHeaderFile(L, SM, HeaderFileExtensions);
+  });
----------------
HerrCai0907 wrote:

I got the point. Our main disagreement is about

> when declaration and definition is in header file, in such case adding static/anonymous namespace is also expected

In my opinion, we should not emit any wrong for the definition in header file. Add static in header file is dangerous and cause potential bug.

e.g.
```c++
/// a.hpp
PREFIX int f() {
  static int a = 1;
  return a++;
}
void b();

/// a.cpp
#include "a.hpp"
#include <iostream>

int main() {
  std::cout << __func__ << " " << f() << "\n";
  b();
  std::cout << __func__ << " " << f() << "\n";
  b();
}

/// b.cpp
#include "a.hpp"
#include <iostream>

void b() { std::cout << __func__ << " " << f() << "\n"; }
```

if `PREFIX` is `inline`, result is `1 2 3 4`.
if `PREFIX` is `static`, result is `1 2 1 2`.
if `PREFIX` is `inline static`, result is `1 2 1 2`.

if `PREFIX` is `template <int>`, result is `1 2 3 4`.
if `PREFIX` is `template <int> static`, result is `1 2 1 2`.

https://github.com/llvm/llvm-project/pull/90830