[clang-tools-extra] [include-cleaner] don't consider the associated header unused (PR #67228)

[kadir çetinkaya via cfe-commits]

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@@ -276,6 +280,27 @@ class PragmaIncludes::RecordPragma : public PPCallbacks, public CommentHandler {
       KeepStack.pop_back(); // Pop immediately for single-line keep pragma.
   }
 
+  // Consider marking H as the "associated header" of the main file.
+  //
+  // Our heuristic:
+  // - it must be the first #include in the main file
+  // - it must have the same name stem as the main file (foo.h and foo.cpp)
+  // (IWYU pragma: associated is also supported, just not by this function).
+  //
+  // We consider the associated header as if it had a keep pragma.
+  // (Unlike IWYU, we don't treat #includes inside the associated header as if
+  // they were written in the main file.)
+  void checkForDeducedAssociated(std::optional<Header> H) {
+    namespace path = llvm::sys::path;
+    if (!InMainFile || SeenAssociatedCandidate)
+      return;
+    SeenAssociatedCandidate = true;
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kadircet wrote:

i think this is still worth a comment, `Make sure we don't consider anything but the first include of the main file` ?

https://github.com/llvm/llvm-project/pull/67228