================
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:`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.


.. _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


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.
  |-- ClangTidyModuleRegistry.h     # Interface for registering of 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
  |-- ObjCModuleTest.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 LLVM`_ 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. For code reviews we mostly use `LLVM Phabricator`_.

.. _Getting Started with LLVM: https://llvm.org/docs/GettingStarted.html
.. _LLVM Coding Standards: https://llvm.org/docs/CodingStandards.html
.. _LLVM Phabricator: https://llvm.org/docs/Phabricator.html

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.), 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.

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 see in more detail at the check class definition:

.. code-block:: c++

  ...

  #include "../ClangTidyCheck.h"

  namespace clang {
  namespace tidy {
  namespace 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;
  };

  } // namespace readability
  } // namespace tidy
  } // namespace clang

  ...

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>`_).


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

(The ``add_new_check.py`` 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/tool/ClangTidyMain.cpp``:

.. 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),
        SomeOption(Options.get("SomeOption1", -1U)),
        SomeOption(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:
    - key: my-check.SomeOption1
      value: 123
    - key: my-check.SomeOption2
      value: '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: [{key: a, value: b}, {key: x, value: y}]}" ...


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

To run tests for :program:`clang-tidy` 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`` by 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
to 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/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;$


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.

.. _lit: https://llvm.org/docs/CommandGuide/lit.html
.. _FileCheck: https://llvm.org/docs/CommandGuide/FileCheck.html
.. _test/clang-tidy/google-readability-casting.cpp: https://reviews.llvm.org/diffusion/L/browse/clang-tools-extra/trunk/test/clang-tidy/google-readability-casting.cpp


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 using the ``-header-filter`` flag. It has the same behavior
  as the corresponding :program:`clang-tidy` flag.

* 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``