[libcxx-commits] [libcxx] [libc++][modules] Adds std.compat module. (PR #71438)

[Louis Dionne via libcxx-commits]

================
@@ -18,15 +18,47 @@ namespace libcpp {
 class header_exportable_declarations : public clang::tidy::ClangTidyCheck {
 public:
   explicit header_exportable_declarations(llvm::StringRef, clang::tidy::ClangTidyContext*);
+  ~header_exportable_declarations();
   void registerMatchers(clang::ast_matchers::MatchFinder*) override;
   void check(const clang::ast_matchers::MatchFinder::MatchResult&) override;
 
-  enum class FileType { Header, ModulePartition, Module, Unknown };
+  enum class FileType {
+    // std module specific
+    Header,
+    CompatModulePartition,
+    Module,
+    // std.compat module specific
+    CHeader,
+    ModulePartition,
+    CompatModule,
+    // invalid value
+    Unknown
+  };
 
 private:
   llvm::StringRef filename_;
   FileType file_type_;
   llvm::StringRef extra_header_;
   std::set<std::string> decls_;
+
+  // The named declarations in .h C headers are "tricky". On POSIX systems
+  // these headers contain POSIX specific functions that do not use a reserved
+  // name. For example, fmemopen is provided by stdio.h.
+  // The way to filter the names that should be provided by the headers is the
+  // following:
+  // - record all named declarations the global namespace
+  // - wait until the header is completely processed
+  // - every named declaration in the global namespace that has a matching
+  //   "export" in the std namespace is exported.
+  //
+  // The only possible place where to process after the header is processed is
+  // the class' destructor.
+  //
+  // It is possible to skip some declarations in the std namespace, these are
+  // added to decls_ before processing. To differenciate between a skipped
+  // declaration and a real declaration the skipped declarations are recorded
+  // in an extra variable.
----------------
ldionne wrote:

```suggestion
  // The only place where we can do the above while ensuring that all the 
  // declarations in the header have been seen is in the clang tidy plugin's
  // destructor.
  //
  // It is possible to skip some declarations in the std namespace, these are
  // added to decls_ before processing. To differentiate between a skipped
  // declaration and a real declaration the skipped declarations are recorded
  // in an extra variable.
```

https://github.com/llvm/llvm-project/pull/71438