[clangd] Do not collect macros when clang-tidy checks call into the preprocessor (#106329)

Fixes https://github.com/llvm/llvm-project/issues/99617

clang-tools-extra/clangd/CollectMacros.cpp

@@ -32,6 +32,7 @@ void CollectMainFileMacros::add(const Token &MacroNameTok, const MacroInfo *MI,
   if (Loc.isInvalid() || Loc.isMacroID())
     return;
 
+  assert(isInsideMainFile(Loc, SM));
   auto Name = MacroNameTok.getIdentifierInfo()->getName();
   Out.Names.insert(Name);
   size_t Start = SM.getFileOffset(Loc);

clang-tools-extra/clangd/CollectMacros.h

@@ -82,6 +82,14 @@ public:
 
   void SourceRangeSkipped(SourceRange R, SourceLocation EndifLoc) override;
 
+  // Called when the AST build is done to disable further recording
+  // of macros by this class. This is needed because some clang-tidy
+  // checks can trigger PP callbacks by calling directly into the
+  // preprocessor. Such calls are not interleaved with FileChanged()
+  // in the expected way, leading this class to erroneously process
+  // macros that are not in the main file.
+  void doneParse() { InMainFile = false; }
+
 private:
   void add(const Token &MacroNameTok, const MacroInfo *MI,
            bool IsDefinition = false, bool InConditionalDirective = false);

clang-tools-extra/clangd/ParsedAST.cpp

@@ -688,7 +688,9 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
     Marks = Patch->marks();
   }
   auto &PP = Clang->getPreprocessor();
-  PP.addPPCallbacks(std::make_unique<CollectMainFileMacros>(PP, Macros));
+  auto MacroCollector = std::make_unique<CollectMainFileMacros>(PP, Macros);
+  auto *MacroCollectorPtr = MacroCollector.get(); // so we can call doneParse()
+  PP.addPPCallbacks(std::move(MacroCollector));
 
   PP.addPPCallbacks(
       collectPragmaMarksCallback(Clang->getSourceManager(), Marks));
@@ -709,6 +711,10 @@ ParsedAST::build(llvm::StringRef Filename, const ParseInputs &Inputs,
     log("Execute() failed when building AST for {0}: {1}", MainInput.getFile(),
         toString(std::move(Err)));
 
+  // Disable the macro collector for the remainder of this function, e.g.
+  // clang-tidy checkers.
+  MacroCollectorPtr->doneParse();
+
   // We have to consume the tokens before running clang-tidy to avoid collecting
   // tokens from running the preprocessor inside the checks (only
   // modernize-use-trailing-return-type does that today).

clang-tools-extra/clangd/unittests/DiagnosticsTests.cpp

@@ -940,6 +940,23 @@ TEST(DiagnosticTest, ClangTidySelfContainedDiagsFormatting) {
                         withFix(equalToFix(ExpectedFix2))))));
 }
 
+TEST(DiagnosticsTest, ClangTidyCallingIntoPreprocessor) {
+  std::string Main = R"cpp(
+    extern "C" {
+    #include "b.h"
+    }
+  )cpp";
+  std::string Header = R"cpp(
+    #define EXTERN extern
+    EXTERN int waldo();
+  )cpp";
+  auto TU = TestTU::withCode(Main);
+  TU.AdditionalFiles["b.h"] = Header;
+  TU.ClangTidyProvider = addTidyChecks("modernize-use-trailing-return-type");
+  // Check that no assertion failures occur during the build
+  TU.build();
+}
+
 TEST(DiagnosticsTest, Preprocessor) {
   // This looks like a preamble, but there's an #else in the middle!
   // Check that: