[clang-tools-extra] [libc] Update libc namespace clang-tidy checks (PR #98424)

[Piotr Zegar via cfe-commits]

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@@ -30,20 +30,35 @@ void ImplementationInNamespaceCheck::check(
   const auto *MatchedDecl =
       Result.Nodes.getNodeAs<Decl>("child_of_translation_unit");
   const auto *NS = dyn_cast<NamespaceDecl>(MatchedDecl);
+
+  // LLVM libc declarations should be inside of a non-anonymous namespace.
   if (NS == nullptr || NS->isAnonymousNamespace()) {
     diag(MatchedDecl->getLocation(),
          "declaration must be enclosed within the '%0' namespace")
-        << RequiredNamespaceMacroName;
+        << RequiredNamespaceDeclMacroName;
     return;
   }
+
+  // Enforce that the namespace is the result of macro expansion
   if (Result.SourceManager->isMacroBodyExpansion(NS->getLocation()) == false) {
     diag(NS->getLocation(), "the outermost namespace should be the '%0' macro")
-        << RequiredNamespaceMacroName;
+        << RequiredNamespaceDeclMacroName;
     return;
   }
-  if (NS->getName().starts_with(RequiredNamespaceStart) == false) {
+
+  // We want the macro to have [[gnu::visibility("hidden")]] as a prefix, but
+  // visibility is just an attribute in the AST construct, so we check that
+  // instead.
+  if (NS->getVisibility() != Visibility::HiddenVisibility) {
     diag(NS->getLocation(), "the '%0' macro should start with '%1'")
-        << RequiredNamespaceMacroName << RequiredNamespaceStart;
+        << RequiredNamespaceDeclMacroName << RequiredNamespaceDeclStart;
+    return;
+  }
+
+  // Lastly, make sure the namespace name actually has the __llvm_libc prefix
+  if (NS->getName().starts_with(RequiredNamespaceRefStart) == false) {
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PiotrZSL wrote:

would be nice to exclude anonymous namespace here, i just fear that getName() could assert

https://github.com/llvm/llvm-project/pull/98424