================
Getting Involved
================

:program:`clang-tidy` has several own checks and can run Clang static analyzer
checks, but its power is in the ability to easily write custom checks.

Checks are organized in modules, which can be linked into :program:`clang-tidy`
with minimal or no code changes in :program:`clang-tidy`.

Checks can plug into the analysis on the preprocessor level using
`PPCallbacks`_ or on the AST level using `AST Matchers`_. When an error is
found, checks can report them in a way similar to how Clang diagnostics work.
A fix-it hint can be attached to a diagnostic message.

The interface provided by :program:`clang-tidy` makes it easy to write useful
and precise checks in just a few lines of code. If you have an idea for a good
check, the rest of this document explains how to do this.

There are a few tools particularly useful when developing clang-tidy checks:
  * ``add_new_check.py`` is a script to automate the process of adding a new
    check; it will create the check, update the CMake file and create a test.
  * ``rename_check.py`` does what the script name suggests, renames an existing
    check.
  * :program:`pp-trace` logs method calls on `PPCallbacks` for a source file
    and is invaluable in understanding the preprocessor mechanism.
  * :program:`clang-query` is invaluable for interactive prototyping of AST
    matchers and exploration of the Clang AST;
  * `clang-check`_ with the ``-ast-dump`` (and optionally ``-ast-dump-filter``)
    provides a convenient way to dump AST of a C++ program.

If CMake is configured with ``CLANG_TIDY_ENABLE_STATIC_ANALYZER=NO``,
:program:`clang-tidy` will not be built with support for the
``clang-analyzer-*`` checks or the ``mpi-*`` checks.

If CMake is configured with ``CLANG_TIDY_ENABLE_QUERY_BASED_CUSTOM_CHECKS=NO``,
:program:`clang-tidy` will not be built with support for query based checks.


.. _AST Matchers: https://clang.llvm.org/docs/LibASTMatchers.html
.. _PPCallbacks: https://clang.llvm.org/doxygen/classclang_1_1PPCallbacks.html
.. _clang-check: https://clang.llvm.org/docs/ClangCheck.html


Choosing the Right Place for your Check
---------------------------------------

If you have an idea of a check, you should decide whether it should be
implemented as a:

+ *Clang diagnostic*: if the check is generic enough, targets code patterns that
  most probably are bugs (rather than style or readability issues), can be
  implemented effectively and with extremely low false-positive rate, it may
  make a good Clang diagnostic.

+ *Clang static analyzer check*: if the check requires some sort of control flow
  analysis, it should probably be implemented as a static analyzer check.

+ *clang-tidy check* is a good choice for linter-style checks, checks that are
  related to a certain coding style, checks that address code readability, etc.


Preparing your Workspace
------------------------

If you are new to LLVM development, you should read the `Getting Started with
the LLVM System`_, `Using Clang Tools`_ and `How To Setup Clang Tooling For
LLVM`_ documents to check out and build LLVM, Clang and Clang Extra Tools with
CMake.

Once you are done, change to the ``llvm/clang-tools-extra`` directory, and
let's start!

.. _Getting Started with the LLVM System: https://llvm.org/docs/GettingStarted.html
.. _Using Clang Tools: https://clang.llvm.org/docs/ClangTools.html
.. _How To Setup Clang Tooling For LLVM: https://clang.llvm.org/docs/HowToSetupToolingForLLVM.html

When you `configure the CMake build <https://llvm.org/docs/GettingStarted.html#local-llvm-configuration>`_,
make sure that you enable the ``clang`` and ``clang-tools-extra`` projects to
build :program:`clang-tidy`.
Because your new check will have associated documentation, you will also want
to install `Sphinx <https://www.sphinx-doc.org/en/master/>`_ and enable it in
the CMake configuration. To save build time of the core Clang libraries, you
may want to only enable the ``X86`` target in the CMake configuration.


The Directory Structure
-----------------------

:program:`clang-tidy` source code resides in the
``llvm/clang-tools-extra`` directory and is structured as follows:

::

  clang-tidy/                       # Clang-tidy core.
  |-- ClangTidy.h                   # Interfaces for users.
  |-- ClangTidyCheck.h              # Interfaces for checks.
  |-- ClangTidyModule.h             # Interface for clang-tidy modules.
     ...
  |-- google/                       # Google clang-tidy module.
  |-+
    |-- GoogleTidyModule.cpp
    |-- GoogleTidyModule.h
          ...
  |-- llvm/                         # LLVM clang-tidy module.
  |-+
    |-- LLVMTidyModule.cpp
    |-- LLVMTidyModule.h
          ...
  |-- objc/                         # Objective-C clang-tidy module.
  |-+
    |-- ObjCTidyModule.cpp
    |-- ObjCTidyModule.h
          ...
  |-- tool/                         # Sources of the clang-tidy binary.
          ...
  test/clang-tidy/                  # Integration tests.
      ...
  unittests/clang-tidy/             # Unit tests.
  |-- ClangTidyTest.h
  |-- GoogleModuleTest.cpp
  |-- LLVMModuleTest.cpp
      ...


Writing a clang-tidy Check
--------------------------

So you have an idea of a useful check for :program:`clang-tidy`.

First, if you're not familiar with LLVM development, read through the `Getting
Started with the LLVM System`_ document for instructions on setting up your
workflow and the `LLVM Coding Standards`_ document to familiarize yourself
with the coding style used in the project. Code reviews for clang-tidy
contributions are done through `LLVM Github`_.

.. _Getting Started with the LLVM System: https://llvm.org/docs/GettingStarted.html
.. _LLVM Coding Standards: https://llvm.org/docs/CodingStandards.html
.. _LLVM Github: https://github.com/llvm/llvm-project

Next, you need to decide which module the check belongs to. Modules
are located in subdirectories of `clang-tidy/
<https://github.com/llvm/llvm-project/tree/main/clang-tools-extra/clang-tidy/>`_
and contain checks targeting a certain aspect of code quality (performance,
readability, etc.), a certain coding style or standard (Google, LLVM, CERT,
etc.) or a widely used API (e.g. MPI). Their names are the same as the
user-facing check group names described :ref:`above <checks-groups-table>`.

After choosing the module and the name for the check, run the
``clang-tidy/add_new_check.py`` script to create the skeleton of the check and
plug it to :program:`clang-tidy`. It's the recommended way of adding new
checks.

By default, the new check will apply only to C++ code. If it should apply under
different language options, use the ``--language`` script's parameter.

If we want to create a `readability-awesome-function-names`, we would run:

.. code-block:: console

  $ clang-tidy/add_new_check.py readability awesome-function-names


The ``add_new_check.py`` script will:
  * create the class for your check inside the specified module's directory and
    register it in the module and in the build system;
  * create a lit test file in the ``test/clang-tidy/`` directory;
  * create a documentation file and include it into the
    ``docs/clang-tidy/checks/list.rst``.

Let's look at the check class definition in more detail:

.. code-block:: c++

  ...

  #include "../ClangTidyCheck.h"

  namespace clang::tidy::readability {

  ...
  class AwesomeFunctionNamesCheck : public ClangTidyCheck {
  public:
    AwesomeFunctionNamesCheck(StringRef Name, ClangTidyContext *Context)
        : ClangTidyCheck(Name, Context) {}
    void registerMatchers(ast_matchers::MatchFinder *Finder) override;
    void check(const ast_matchers::MatchFinder::MatchResult &Result) override;
    bool isLanguageVersionSupported(const LangOptions &LangOpts) const override {
      return LangOpts.CPlusPlus;
    }
  };

  } // namespace clang::tidy::readability

  ...

Constructor of the check receives the ``Name`` and ``Context`` parameters, and
must forward them to the ``ClangTidyCheck`` constructor.

In our case the check needs to operate on the AST level and it overrides the
``registerMatchers`` and ``check`` methods. If we wanted to analyze code on the
preprocessor level, we'd need instead to override the ``registerPPCallbacks``
method.

In the ``registerMatchers`` method, we create an AST Matcher (see `AST
Matchers`_ for more information) that will find the pattern in the AST that we
want to inspect. The results of the matching are passed to the ``check`` method,
which can further inspect them and report diagnostics.

.. code-block:: c++

  using namespace ast_matchers;

  void AwesomeFunctionNamesCheck::registerMatchers(MatchFinder *Finder) {
    Finder->addMatcher(functionDecl().bind("x"), this);
  }

  void AwesomeFunctionNamesCheck::check(const MatchFinder::MatchResult &Result) {
    const auto *MatchedDecl = Result.Nodes.getNodeAs<FunctionDecl>("x");
    if (!MatchedDecl->getIdentifier() || MatchedDecl->getName().startswith("awesome_"))
      return;
    diag(MatchedDecl->getLocation(), "function %0 is insufficiently awesome")
        << MatchedDecl
        << FixItHint::CreateInsertion(MatchedDecl->getLocation(), "awesome_");
  }

(If you want to see an example of a useful check, look at
`clang-tidy/google/ExplicitConstructorCheck.h
<https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/clang-tidy/google/ExplicitConstructorCheck.h>`_
and `clang-tidy/google/ExplicitConstructorCheck.cpp
<https://reviews.llvm.org/diffusion/L/browse/clang-tools-extra/trunk/clang-tidy/google/ExplicitConstructorCheck.cpp>`_).

If you need to interact with macros or preprocessor directives, you will want
to override the method ``registerPPCallbacks``. The ``add_new_check.py``
script does not generate an override for this method in the starting point for
your new check.

Check development tips
----------------------

Writing your first check can be a daunting task, particularly if you are
unfamiliar with the LLVM and Clang code bases. Here are some suggestions for
orienting yourself in the codebase and working on your check incrementally.

Guide to useful documentation
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Many of the support classes created for LLVM are used by Clang, such as `StringRef
<https://llvm.org/docs/ProgrammersManual.html#the-stringref-class>`_
and `SmallVector <https://llvm.org/docs/ProgrammersManual.html#llvm-adt-smallvector-h>`_.
These and other commonly used classes are described in the `Important and
useful LLVM APIs <https://llvm.org/docs/ProgrammersManual.html#important-and-useful-llvm-apis>`_
and `Picking the Right Data Structure for the Task
<https://llvm.org/docs/ProgrammersManual.html#picking-the-right-data-structure-for-a-task>`_
sections of the `LLVM Programmer's Manual
<https://llvm.org/docs/ProgrammersManual.html>`. You don't need to memorize all the
details of these classes; the generated `doxygen documentation <https://llvm.org/doxygen/>`_
has everything if you need it. In the header `LLVM/ADT/STLExtras.h
<https://llvm.org/doxygen/STLExtras_8h.html>`_ you'll find useful versions of the STL
algorithms that operate on LLVM containers, such as `llvm::all_of
<https://llvm.org/doxygen/STLExtras_8h.html#func-members>`_.

Clang is implemented on top of LLVM and introduces its own set of classes that
you will interact with while writing your check. When a check issues
diagnostics and fix-its, these are associated with locations in the source
code. Source code locations, source files, ranges of source locations and the
`SourceManager <https://clang.llvm.org/doxygen/classclang_1_1SourceManager.html>`_
class provide the mechanisms for describing such locations. These and
other topics are described in the `"Clang" CFE Internals Manual
<https://clang.llvm.org/docs/InternalsManual.html>`_. Whereas the doxygen generated
documentation serves as a reference to the internals of Clang, this document
serves as a guide to other developers. Topics in that manual of interest to a
check developer are:

- `The Clang "Basic" Library
  <https://clang.llvm.org/docs/InternalsManual.html#the-clang-basic-library>`_ for
  information about diagnostics, fix-it hints and source locations.
- `The Lexer and Preprocessor Library
  <https://clang.llvm.org/docs/InternalsManual.html#the-lexer-and-preprocessor-library>`_
  for information about tokens, lexing (transforming characters into tokens)
  and the preprocessor.
- `The AST Library
  <https://clang.llvm.org/docs/InternalsManual.html#the-ast-library>`_
  for information about how C++ source statements are represented as an
  abstract syntax tree (AST).

Most checks will interact with C++ source code via the AST. Some checks will
interact with the preprocessor. The input source file is lexed and preprocessed
and then parsed into the AST. Once the AST is fully constructed, the check is
run by applying the check's registered AST matchers against the AST and
invoking the check with the set of matched nodes from the AST. Monitoring the
actions of the preprocessor is detached from the AST construction, but a check
can collect information during preprocessing for later use by the check when
nodes are matched by the AST.

Every syntactic (and sometimes semantic) element of the C++ source code is
represented by different classes in the AST. You select the portions of the
AST you're interested in by composing AST matcher functions. You will want
to study carefully the `AST Matcher Reference
<https://clang.llvm.org/docs/LibASTMatchersReference.html>`_
to understand the relationship between the different matcher functions.

Using the Transformer library
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The Transformer library allows you to write a check that transforms source code
by expressing the transformation as a ``RewriteRule``. The Transformer library
provides functions for composing edits to source code to create rewrite rules.
Unless you need to perform low-level source location manipulation, you may want
to consider writing your check with the Transformer library. The `Clang
Transformer Tutorial <https://clang.llvm.org/docs/ClangTransformerTutorial.html>`_
describes the Transformer library in detail.

To use the Transformer library, make the following changes to the code
generated by the ``add_new_check.py`` script:

- Include ``../utils/TransformerClangTidyCheck.h`` instead of
  ``../ClangTidyCheck.h``
- Change the base class of your check from ``ClangTidyCheck`` to
  ``TransformerClangTidyCheck``
- Delete the override of the ``registerMatchers`` and ``check`` methods in
  your check class.
- Write a function that creates the ``RewriteRule`` for your check.
- Call the function in your check's constructor to pass the rewrite rule to
  ``TransformerClangTidyCheck``'s constructor.

Developing your check incrementally
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The best way to develop your check is to start with simple test cases and
increase complexity incrementally. The test file created by the
``add_new_check.py`` script is a starting point for your test cases.
A rough outline of the process looks like this:

- Write a test case for your check.
- Prototype matchers on the test file using :program:`clang-query`.
- Capture the working matchers in the ``registerMatchers`` method.
- Issue the necessary diagnostics and fix-its in the ``check`` method.
- Add the necessary ``CHECK-MESSAGES`` and ``CHECK-FIXES`` annotations to your
  test case to validate the diagnostics and fix-its.
- Build the target ``check-clang-tools`` to confirm the test passes.
- Repeat the process until all aspects of your check are covered by tests.

The quickest way to prototype your matcher is to use :program:`clang-query` to
interactively build up your matcher. For complicated matchers, build up a
matching expression incrementally and use :program:`clang-query`'s ``let``
command to save named matching expressions to simplify your matcher.

.. code-block:: console

  clang-query> let c1 cxxRecordDecl()
  clang-query> match c1

Alternatively, pressing the tab key after a previous matcher's open parentheses
would also show which matchers can be chained with the previous matcher,
though some matchers that work may not be listed. Note that tab completion
does not currently work on Windows.

Just like breaking up a huge function into smaller chunks with
intention-revealing names can help you understand a complex algorithm, breaking
up a matcher into smaller matchers with intention-revealing names can help
you understand a complicated matcher.

Once you have a working :program:`clang-query` matcher, the C++ API matchers
will be the same or similar to your interactively constructed matcher (there
can be cases where they differ slightly). You can use local variables to
preserve your intention-revealing names that you applied to nested matchers.

Creating private matchers
^^^^^^^^^^^^^^^^^^^^^^^^^

Sometimes you want to match a specific aspect of the AST that isn't provided
by the existing AST matchers. You can create your own private matcher using
the same infrastructure as the public matchers. A private matcher can
simplify the processing in your ``check`` method by eliminating complex
hand-crafted AST traversal of the matched nodes. Using the private matcher
allows you to select the desired portions of the AST directly in the matcher
and refer to it by a bound name in the ``check`` method.

Unit testing helper code
^^^^^^^^^^^^^^^^^^^^^^^^

Private custom matchers are a good example of auxiliary support code for your
check that can be tested with a unit test. It will be easier to test your
matchers or other support classes by writing a unit test than by writing a
``FileCheck`` integration test. The ``ASTMatchersTests`` target contains unit
tests for the public AST matcher classes and is a good source of testing
idioms for matchers.

You can build the Clang-Tidy unit tests by building the ``ClangTidyTests``
target. Test targets in LLVM and Clang are excluded from the "build all" style
action of IDE-based CMake generators, so you need to explicitly build the
target for the unit tests to be built.

Making your check robust
^^^^^^^^^^^^^^^^^^^^^^^^

Once you've covered your check with the basic "happy path" scenarios, you'll
want to torture your check with as many edge cases as you can cover in order to
ensure your check is robust. Running your check on a large code base, such as
Clang/LLVM, is a good way to catch things you forgot to account for in your
matchers. However, the LLVM code base may be insufficient for testing purposes
as it was developed against a particular set of coding styles and quality
measures. The larger the corpus of code the check is tested against, the
higher confidence the community will have in the check's efficacy and
false-positive rate.

Some suggestions to ensure your check is robust:

- Create header files that contain code matched by your check.
- Validate that fix-its are properly applied to test header files with
  :program:`clang-tidy`. You will need to perform this test manually until
  automated support for checking messages and fix-its is added to the
  ``check_clang_tidy.py`` script.
- Define macros that contain code matched by your check.
- Define template classes that contain code matched by your check.
- Define template specializations that contain code matched by your check.
- Test your check under both Windows and Linux environments. For example, when
  a fix-it inserts new lines, use the source file's existing newline style
  instead of hard-coding ``\n``. You can use
  ``SourceManager::getBufferData(FileID).detectEOL()`` to get the newline style
  for a file.
- Watch out for high false-positive rates. Ideally, a check would have no
  false positives, but given that matching against an AST is not control-
  or data flow- sensitive, a number of false positives are expected. The
  higher the false-positive rate, the less likely the check will be adopted
  in practice. Mechanisms should be put in place to help the user manage
  false positives.
- There are two primary mechanisms for managing false positives: supporting a
  code pattern which allows the programmer to silence the diagnostic in an ad
  hoc manner and check configuration options to control the behavior of the
  check.
- Consider supporting a code pattern to allow the programmer to silence the
  diagnostic whenever such a code pattern can clearly express the programmer's
  intent. For example, allowing an explicit cast to ``void`` to silence an
  unused variable diagnostic.
- Consider adding check configuration options to allow the user to opt into
  more aggressive checking behavior without burdening users for the common
  high-confidence cases.

Documenting your check
^^^^^^^^^^^^^^^^^^^^^^

The ``add_new_check.py`` script creates entries in the
`release notes <https://clang.llvm.org/extra/ReleaseNotes.html>`_, the list of

checks and a new file for the check documentation itself. It is recommended
that you have a concise summary of what your check does in a single sentence
that is repeated in the release notes, as the first sentence in the doxygen
comments in the header file for your check class and as the first sentence of
the check documentation. Avoid the phrase "this check" in your check summary
and check documentation.

If your check relates to a published coding guideline (C++ Core Guidelines,
SEI CERT, etc.) or style guide, provide links to the relevant guideline or
style guide sections in your check documentation.

Provide enough examples of the diagnostics and fix-its provided by the check so
that a user can easily understand what will happen to their code when the check
is run. If there are exceptions or limitations to your check, document them
thoroughly. This will help users understand the scope of the diagnostics and
fix-its provided by the check.

Building the target ``docs-clang-tools-html`` will run the Sphinx documentation
generator and create HTML documentation files in the
``tools/clang/tools/extra/docs/html`` directory in your build tree.
Make sure that your check is correctly shown in the release notes and the list
of checks. Make sure that the formatting and structure of your check's
documentation look correct: there is no trailing whitespaces and lines are no
longer than 80 characters.

To validate your files, please use ``doc8`` as described below.

Clang-Tidy uses `doc8 <https://pypi.org/project/doc8/>`_ to check ``.rst``
files for formatting consistency. You can install ``doc8`` with ``pip``:

.. code-block:: console

  $ pip install doc8

To run ``doc8`` on the modified documentations:

.. code-block:: console

  $ git diff --name-only HEAD -- clang-tools-extra/docs/clang-tidy/ | grep "\.rst$" | xargs -r doc8


Registering your Check
----------------------

(The ``add_new_check.py`` script takes care of registering the check in an
existing module. If you want to create a new module or know the details,
read on.)

The check should be registered in the corresponding module with a distinct
name:

.. code-block:: c++

  class MyModule : public ClangTidyModule {
   public:
    void addCheckFactories(ClangTidyCheckFactories &CheckFactories) override {
      CheckFactories.registerCheck<ExplicitConstructorCheck>(
          "my-explicit-constructor");
    }
  };

Now we need to register the module in the ``ClangTidyModuleRegistry`` using a
statically initialized variable:

.. code-block:: c++

  static ClangTidyModuleRegistry::Add<MyModule> X("my-module",
                                                  "Adds my lint checks.");


When using LLVM build system, we need to use the following hack to ensure the
module is linked into the :program:`clang-tidy` binary:

Add this near the ``ClangTidyModuleRegistry::Add<MyModule>`` variable:

.. code-block:: c++

  // This anchor is used to force the linker to link in the generated object file
  // and thus register the MyModule.
  volatile int MyModuleAnchorSource = 0;

And this to the main translation unit of the :program:`clang-tidy` binary (or
the binary you link the ``clang-tidy`` library in)
``clang-tidy/ClangTidyForceLinker.h``:

.. code-block:: c++

  // This anchor is used to force the linker to link the MyModule.
  extern volatile int MyModuleAnchorSource;
  static int MyModuleAnchorDestination = MyModuleAnchorSource;


Configuring Checks
------------------

If a check needs configuration options, it can access check-specific options
using the ``Options.get<Type>("SomeOption", DefaultValue)`` call in the check
constructor. In this case, the check should also override the
``ClangTidyCheck::storeOptions`` method to make the options provided by the
check discoverable. This method lets :program:`clang-tidy` know which options
the check implements and what the current values are (e.g. for the
``-dump-config`` command-line option).

.. code-block:: c++

  class MyCheck : public ClangTidyCheck {
    const unsigned SomeOption1;
    const std::string SomeOption2;

  public:
    MyCheck(StringRef Name, ClangTidyContext *Context)
      : ClangTidyCheck(Name, Context),
        SomeOption1(Options.get("SomeOption1", -1U)),
        SomeOption2(Options.get("SomeOption2", "some default")) {}

    void storeOptions(ClangTidyOptions::OptionMap &Opts) override {
      Options.store(Opts, "SomeOption1", SomeOption1);
      Options.store(Opts, "SomeOption2", SomeOption2);
    }
    ...

Assuming the check is registered with the name "my-check", the option can then
be set in a ``.clang-tidy`` file in the following way:

.. code-block:: yaml

  CheckOptions:
    my-check.SomeOption1: 123
    my-check.SomeOption2: 'some other value'

If you need to specify check options on a command line, you can use the inline
YAML format:

.. code-block:: console

  $ clang-tidy -config="{CheckOptions: {a: b, x: y}}" ...


Testing Checks
--------------

To run tests for :program:`clang-tidy`, build the ``check-clang-tools``
target. For instance, if you configured your CMake build with the ninja project
generator, use the command:

.. code-block:: console

  $ ninja check-clang-tools

:program:`clang-tidy` checks can be tested using either unit tests or
`lit`_ tests. Unit tests may be more convenient to test complex replacements
with strict checks. `Lit`_ tests allow using partial text matching and regular
expressions which makes them more suitable for writing compact tests for
diagnostic messages.

The ``check_clang_tidy.py`` script provides an easy way to test both
diagnostic messages and fix-its. It filters out ``CHECK`` lines from the test
file, runs :program:`clang-tidy` and verifies messages and fixes with two
separate `FileCheck`_ invocations: once with FileCheck's directive
prefix set to ``CHECK-MESSAGES``, validating the diagnostic messages,
and once with the directive prefix set to ``CHECK-FIXES``, running
against the fixed code (i.e., the code after generated fix-its are
applied). In particular, ``CHECK-FIXES:`` can be used to check
that code was not modified by fix-its, by checking that it is present
unchanged in the fixed code. The full set of `FileCheck`_ directives
is available (e.g., ``CHECK-MESSAGES-SAME:``, ``CHECK-MESSAGES-NOT:``), though
typically the basic ``CHECK`` forms (``CHECK-MESSAGES`` and ``CHECK-FIXES``)
are sufficient for clang-tidy tests. Note that the `FileCheck`_
documentation mostly assumes the default prefix (``CHECK``), and hence
describes the directive as ``CHECK:``, ``CHECK-SAME:``, ``CHECK-NOT:``, etc.
Replace ``CHECK`` with either ``CHECK-FIXES`` or ``CHECK-MESSAGES`` for
clang-tidy tests.

An additional check enabled by ``check_clang_tidy.py`` ensures that
if `CHECK-MESSAGES:` is used in a file then every warning or error
must have an associated CHECK in that file. Or, you can use ``CHECK-NOTES:``
instead, if you want to **also** ensure that all the notes are checked.

To use the ``check_clang_tidy.py`` script, put a .cpp file with the
appropriate ``RUN`` line in the ``test/clang-tidy`` directory. Use
``CHECK-MESSAGES:`` and ``CHECK-FIXES:`` lines to write checks against
diagnostic messages and fixed code.

It's advised to make the checks as specific as possible to avoid checks
matching incorrect parts of the input. Use ``[[@LINE+X]]``/``[[@LINE-X]]``
substitutions and distinct function and variable names in the test code.

Here's an example of a test using the ``check_clang_tidy.py`` script (the full
source code is at `test/clang-tidy/checkers/google/readability-casting.cpp`_):

.. code-block:: c++

  // RUN: %check_clang_tidy %s google-readability-casting %t

  void f(int a) {
    int b = (int)a;
    // CHECK-MESSAGES: :[[@LINE-1]]:11: warning: redundant cast to the same type [google-readability-casting]
    // CHECK-FIXES: int b = a;
  }

To check more than one scenario in the same test file, use
``-check-suffix=SUFFIX-NAME`` on ``check_clang_tidy.py`` command line or
``-check-suffixes=SUFFIX-NAME-1,SUFFIX-NAME-2,...``.
With ``-check-suffix[es]=SUFFIX-NAME`` you need to replace your ``CHECK-*``
directives with ``CHECK-MESSAGES-SUFFIX-NAME`` and ``CHECK-FIXES-SUFFIX-NAME``.

Here's an example:

.. code-block:: c++

   // RUN: %check_clang_tidy -check-suffix=USING-A %s misc-unused-using-decls %t -- -- -DUSING_A
   // RUN: %check_clang_tidy -check-suffix=USING-B %s misc-unused-using-decls %t -- -- -DUSING_B
   // RUN: %check_clang_tidy %s misc-unused-using-decls %t
   ...
   // CHECK-MESSAGES-USING-A: :[[@LINE-8]]:10: warning: using decl 'A' {{.*}}
   // CHECK-MESSAGES-USING-B: :[[@LINE-7]]:10: warning: using decl 'B' {{.*}}
   // CHECK-MESSAGES: :[[@LINE-6]]:10: warning: using decl 'C' {{.*}}
   // CHECK-FIXES-USING-A-NOT: using a::A;$
   // CHECK-FIXES-USING-B-NOT: using a::B;$
   // CHECK-FIXES-NOT: using a::C;$

The ``-std`` flag controls which C or C++ standard(s) the test is compiled
under. It accepts a comma-separated list of standards and supports an
``-or-later`` suffix:

- ``-std=c++17``: runs the test **only** with C++17.
- ``-std=c++17-or-later``: runs the test once for each standard from C++17
  onwards (currently C++17, C++20, C++23, and C++26), in separate invocations.
  Use this when a check should work correctly on all modern standards.
- ``-std=c++14,c++17``: runs the test once with C++14 and once with C++17.

When no ``-std`` is given, ``check_clang_tidy.py`` defaults to
``c++11-or-later`` for C++ files and ``c99-or-later`` for C files. The check
skeleton generated by ``add_new_check.py`` uses the ``-or-later`` form by
default. Prefer ``-std=<minimum>-or-later`` unless the test expects
behavior that only applies to a specific standard version.

There are many dark corners in the C++ language, and it may be difficult to
make your check work perfectly in all cases, especially if it issues fix-it
hints. The most frequent pitfalls are macros and templates:

1. Code written in a macro body/template definition may have a different
   meaning depending on the macro expansion/template instantiation.
2. Multiple macro expansions/template instantiations may result in the
   same code being inspected by the check multiple times (possibly, with
   different meanings, see 1), and the same warning (or a slightly different
   one) may be issued by the check multiple times; :program:`clang-tidy` will
   deduplicate _identical_ warnings, but if the warnings are slightly
   different, all of them will be shown to the user (and used for applying
   fixes, if any).
3. Making replacements to a macro body/template definition may be fine for
   some macro expansions/template instantiations, but easily break some other
   expansions/instantiations.

If you need multiple files to exercise all the aspects of your check, it is
recommended you place them in a subdirectory named for the check under the
``Inputs`` directory for the module containing your check. This keeps the
test directory from getting cluttered.

If you need to validate how your check interacts with system header files, a
set of simulated system header files is located in the
``checkers/Inputs/Headers`` directory. The path to this directory is
available in a lit test with the variable ``%clang_tidy_headers``.

.. _lit: https://llvm.org/docs/CommandGuide/lit.html
.. _FileCheck: https://llvm.org/docs/CommandGuide/FileCheck.html
.. _test/clang-tidy/checkers/google/readability-casting.cpp: https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/test/clang-tidy/checkers/google/readability-casting.cpp


Submitting a Pull Request
-------------------------

Before submitting a pull request, contributors are encouraged to run
:program:`clang-tidy` and :program:`clang-format` on their changes to ensure
code quality and catch potential issues. While :program:`clang-tidy` is not
currently enforced in CI, following this practice helps maintain code
consistency and prevent common errors.

Here's a useful command to check your staged changes:

.. code-block:: console

  $ git diff --staged -U0 | ./clang-tools-extra/clang-tidy/tool/clang-tidy-diff.py \
      -j $(nproc) -path build/ -p1 -only-check-in-db
  $ git clang-format

Note that some warnings may be false positives or require careful consideration
before fixing. Use your judgment and feel free to discuss in the pull request
if you're unsure about a particular warning.


Out-of-tree check plugins
-------------------------


Developing an out-of-tree check as a plugin largely follows the steps
outlined above, including creating a new module and doing the hacks to
register the module. The plugin is a shared library whose code lives outside
the clang-tidy build system. Build and link this shared library against
LLVM as done for other kinds of Clang plugins. If using CMake, use the keyword
``MODULE`` while invoking ``add_library`` or ``llvm_add_library``.

The plugin can be loaded by passing `-load` to `clang-tidy` in addition to the
names of the checks to enable.

.. code-block:: console

  $ clang-tidy --checks=-*,my-explicit-constructor -list-checks -load myplugin.so

There are no expectations regarding ABI and API stability, so the plugin must
be compiled against the version of clang-tidy that will be loading the plugin.

The plugins can use threads, TLS, or any other facilities available to in-tree
code which is accessible from the external headers.

Note that testing out-of-tree checks might involve getting ``llvm-lit`` from an
LLVM installation compiled from source. See `Getting Started with the LLVM
System`_ for ways to do so.

Alternatively, get `lit`_ following the `test-suite guide`_ and get the
`FileCheck`_ binary, and write a version of `check_clang_tidy.py`_ to suit your
needs.

.. _Getting Started with the LLVM System: https://llvm.org/docs/GettingStarted.html
.. _test-suite guide: https://llvm.org/docs/TestSuiteGuide.html
.. _lit: https://llvm.org/docs/CommandGuide/lit.html
.. _FileCheck: https://llvm.org/docs/CommandGuide/FileCheck.html
.. _check_clang_tidy.py: https://github.com/llvm/llvm-project/blob/main/clang-tools-extra/test/clang-tidy/check_clang_tidy.py

Running clang-tidy on LLVM
--------------------------

To test a check, it's best to try it out on a larger code base. LLVM and Clang
are the natural targets as you already have the source code around. The most
convenient way to run :program:`clang-tidy` is with a compile command database;
CMake can automatically generate one; for a description of how to enable it,
see `How To Setup Clang Tooling For LLVM`_. Once ``compile_commands.json`` is
in place and a working version of :program:`clang-tidy` is in ``PATH`` the
entire code base can be analyzed with ``clang-tidy/tool/run-clang-tidy.py``.
The script executes :program:`clang-tidy` with the default set of checks on
every translation unit in the compile command database and displays the
resulting warnings and errors. The script provides multiple configuration
flags.

.. _How To Setup Clang Tooling For LLVM: https://clang.llvm.org/docs/HowToSetupToolingForLLVM.html


* The default set of checks can be overridden using the ``-checks`` argument,
  taking the identical format as :program:`clang-tidy` does. For example,
  ``-checks=-*,modernize-use-override`` will run the ``modernize-use-override``
  check only.

* To restrict the files examined, you can provide one or more regex arguments
  that the file names are matched against.
  ``run-clang-tidy.py clang-tidy/.*Check\.cpp`` will only analyze `clang-tidy`
  checks. It may also be necessary to restrict the header files that warnings
  are displayed from by using the ``-header-filter`` and
  ``-exclude-header-filter`` flags. They have the same behavior as the
  corresponding :program:`clang-tidy` flags.

* To apply suggested fixes, ``-fix`` can be passed as an argument. This gathers
  all changes in a temporary directory and applies them. Passing ``-format``
  will run clang-format over changed lines.


On checks profiling
-------------------

:program:`clang-tidy` can collect per-check profiling info, and output it
for each processed source file (translation unit).

To enable profiling info collection, use the ``-enable-check-profile``
argument. The timings will be output to ``stderr`` as a table.
Example output:

.. code-block:: console

  $ clang-tidy -enable-check-profile -checks=-*,readability-function-size source.cpp
  ===-------------------------------------------------------------------------===
                            clang-tidy checks profiling
  ===-------------------------------------------------------------------------===
    Total Execution Time: 1.0282 seconds (1.0258 wall clock)

     ---User Time---   --System Time--   --User+System--   ---Wall Time---  --- Name ---
     0.9136 (100.0%)   0.1146 (100.0%)   1.0282 (100.0%)   1.0258 (100.0%)  readability-function-size
     0.9136 (100.0%)   0.1146 (100.0%)   1.0282 (100.0%)   1.0258 (100.0%)  Total

It can also store that data as JSON files for further processing.
Example output:

.. code-block:: console

  $ clang-tidy -enable-check-profile -store-check-profile=. -checks=-*,readability-function-size source.cpp
  $ # Note that there won't be timings table printed to the console.
  $ ls /tmp/out/
  20180516161318717446360-source.cpp.json
  $ cat 20180516161318717446360-source.cpp.json
  {
  "file": "/path/to/source.cpp",
  "timestamp": "2018-05-16 16:13:18.717446360",
  "profile": {
    "time.clang-tidy.readability-function-size.wall": 1.0421266555786133e+00,
    "time.clang-tidy.readability-function-size.user": 9.2088400000005421e-01,
    "time.clang-tidy.readability-function-size.sys": 1.2418899999999974e-01
  }
  }

There is only one argument that controls profile storage:

* ``-store-check-profile=<prefix>``

  By default, reports are printed in tabulated format to stderr. When this
  option is passed, these per-TU profiles are instead stored as JSON.
  If the prefix is not an absolute path, it is considered to be relative to the
  directory from where you have run :program:`clang-tidy`. All ``.`` and ``..``
  patterns in the path are collapsed, and symlinks are resolved.

  Example:
  Let's suppose you have a source file named ``example.cpp``, located in the
  ``/source`` directory. Only the input filename is used, not the full path
  to the source file. Additionally, it is prefixed with the current timestamp.

  * If you specify ``-store-check-profile=/tmp``, then the profile will be
    saved to ``/tmp/<ISO8601-like timestamp>-example.cpp.json``

  * If you run :program:`clang-tidy` from within ``/foo`` directory, and
    specify ``-store-check-profile=.``, then the profile will still be
    saved to ``/foo/<ISO8601-like timestamp>-example.cpp.json``
