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clang-tidy

Important clang-tidy points are emphasized here. Read this if you're planning on changing how we use clang-tidy.

Quick warning: don't trust the internet for examples on how to use clang-tidy. Most projects mislead you by having typos, using non-existent features or messing up in a way that disables all the checks. The official documentation and source code are your best bet.

Overview

1. clang-tidy is a static analyzer and linter. It checks for common code smells.
2. When you run clang-tidy, it searches parent directories for a .clang-tidy file.
   • clang-tidy -dump-config will print out the current configuration.
3. clang-tidy reads the output of compile_commands.json, which is generated when you run cmake.
   1. compile_commands.json only contains .cpp files.
   2. Therefore, clang-tidy can only process C++ (.cpp) files. It cannot process header (.h) files by themselves.
   3. clang-tidy will however process .h files which are included in a .cpp file.
   4. You might not want warnings from all header files, e.g. only src/, not third_party/.
   5. clang-tidy only supports whitelisting headers via a single regex string.
      • HeaderFilterRegex: 'REGEX' in .clang-tidy
      • -header-filter=REGEX on the command line, note that this overrides your .clang-tidy setting.
      • The (undocumented) format of the regex is POSIX ERE. See the implementation of HeaderFilterRegex and llvm::Regex.
4. clang-tidy separates the notion of Warning, Error, Compilation Error.
   • clang-tidy will only display warnings for enabled checks.
   • clang-tidy will convert all warnings that match the WarningsAsErrors regex to errors.
     • WarningsAsErrors does not enable any checks on its own.
   • The list of checks are parsed as regex. If you make a typo, it silently ignores that check.
   • Run clang-tidy -list-checks to confirm which checks are enabled.
5. You can get clang-tidy to leave certain lines alone.
   • To ignore the same line, put // NOLINT at the end
   • To ignore the next line, put // NOLINTNEXTLINE on the immediate preceding line
   • Wherever possible, avoid using line-filter. Nobody wants to maintain line numbers when code gets added/deleted.
   • A clang-diagnostic-error may mean a compilation problem. No amount of NOLINT or disabling checks will shut that up. If you're pulling in third-party header dependencies, make sure they're a dependency for our check-clang-tidy make target too.

Gotchas

1. Running clang-tidy on a list of files will run clang-tidy on each file sequentially.
2. If you include a header file which needs fixing in multiple .cpp files, clang-tidy will repeat the fix multiple times. An example which was included in three files:

Original code : if (last_errno_)
"Fixed" code  : if (last_errno_ != 0 != 0 != 0)

For both of the above problems, LLVM recommends using their run-clang-tidy.py script. It will gather all the code fixes and apply them at once to prevent this issue. It also supports parallelism.

However, if you use LLVM's default run-clang-tidy.py, caveat emptor:

• It doesn't work on Python 3.
• It will override .clang-tidy's HeaderFilterRegex.
• From a Pythonic/PyLint point of view, the code quality isn't great.
• That said, it is actively being developed and the above criticisms may no longer be valid.

We currently use a modified version of run-clang-tidy.py.