[clang-tools-extra] [clangd] Extend FeatureModule hooks (PR #221054)

[Aleksandr Platonov via cfe-commits]

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@@ -86,19 +125,48 @@ TEST(FeatureModulesTest, SuppressDiags) {
   }
 }
 
+TEST(FeatureModulesTest, BeforePPCallbacks) {
+  struct IncludeRecorder : public PPCallbacks {
+    IncludeRecorder(std::vector<std::string> &Includes) : Includes(Includes) {}
+
+    void InclusionDirective(SourceLocation, const Token &, StringRef FileName,
+                            bool, CharSourceRange, OptionalFileEntryRef,
+                            StringRef, StringRef, const clang::Module *, bool,
+                            SrcMgr::CharacteristicKind) override {
+      Includes.push_back(FileName.str());
+    }
+
+  private:
+    std::vector<std::string> &Includes;
+  };
+  std::vector<std::string> Includes;
+  auto Module = std::make_unique<TestModule>();
+  Module->BeforePPCallbacks = [&Includes](CompilerInstance &CI) {
+    // The preamble build sees this include directly. Register only during the
+    // main-file build to verify the callback sees the replayed event.
----------------
ArcsinX wrote:

> But where I get lost here is what does the preamble build see, in my naive world it only looks at the includes themselves and not the main file at all,

This is right, but when processing the main file, we still need to observe the include directives, but we cannot do so directly due to preamble optimization. To address this, the preamble replay mechanism simulates real include directive processing by replaying preamble events and triggering preprocessor callbacks during the main file re-parse (even though the PCH is loaded instead).This test verifies that our extension to the feature module interface correctly exposes preprocessor events via the preamble replay mechanism within a feature module during main file processing. (ProgramAction == frontend::ParseSyntaxOnly indicates main file processing, whereas PCH generation uses the GeneratePCH action)

For clang-tidy this opens ability to use checks which relies on preprocessor events (e.g. llvm-header-guard)

https://github.com/llvm/llvm-project/pull/221054