src/third_party/llvm-project/clang-tools-extra/docs/include-fixer.rst

===================
Clang-Include-Fixer
===================

.. contents::

One of the major nuisances of C++ compared to other languages is the manual
management of ``#include`` directives in any file.
:program:`clang-include-fixer` addresses one aspect of this problem by providing
an automated way of adding ``#include`` directives for missing symbols in one
translation unit.

While inserting missing ``#include``, :program:`clang-include-fixer` adds
missing namespace qualifiers to all instances of an unidentified symbol if
the symbol is missing some prefix namespace qualifiers.

Setup
=====

To use :program:`clang-include-fixer` two databases are required. Both can be
generated with existing tools.

- Compilation database. Contains the compiler commands for any given file in a
  project and can be generated by CMake, see `How To Setup Tooling For LLVM`_.
- Symbol index. Contains all symbol information in a project to match a given
  identifier to a header file.

Ideally both databases (``compile_commands.json`` and
``find_all_symbols_db.yaml``) are linked into the root of the source tree they
correspond to. Then the :program:`clang-include-fixer` can automatically pick
them up if called with a source file from that tree. Note that by default
``compile_commands.json`` as generated by CMake does not include header files,
so only implementation files can be handled by tools.

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

Creating a Symbol Index From a Compilation Database
---------------------------------------------------

The include fixer contains :program:`find-all-symbols`, a tool to create a
symbol database in YAML format from a compilation database by parsing all
source files listed in it. The following list of commands shows how to set up a
database for LLVM, any project built by CMake should follow similar steps.

.. code-block:: console

  $ cd path/to/llvm-build
  $ ninja find-all-symbols // build find-all-symbols tool.
  $ ninja clang-include-fixer // build clang-include-fixer tool.
  $ ls compile_commands.json # Make sure compile_commands.json exists.
    compile_commands.json
  $ path/to/llvm/source/tools/clang/tools/extra/include-fixer/find-all-symbols/tool/run-find-all-symbols.py
    ... wait as clang indexes the code base ...
  $ ln -s $PWD/find_all_symbols_db.yaml path/to/llvm/source/ # Link database into the source tree.
  $ ln -s $PWD/compile_commands.json path/to/llvm/source/ # Also link compilation database if it's not there already.
  $ cd path/to/llvm/source
  $ /path/to/clang-include-fixer -db=yaml path/to/file/with/missing/include.cpp
    Added #include "foo.h"

Integrate with Vim
------------------
To run `clang-include-fixer` on a potentially unsaved buffer in Vim. Add the
following key binding to your ``.vimrc``:

.. code-block:: console

  noremap <leader>cf :pyf path/to/llvm/source/tools/clang/tools/extra/include-fixer/tool/clang-include-fixer.py<cr>

This enables `clang-include-fixer` for NORMAL and VISUAL mode. Change
`<leader>cf` to another binding if you need clang-include-fixer on a different
key. The `<leader> key
<http://vim.wikia.com/wiki/Mapping_keys_in_Vim_-_Tutorial_(Part_3)#Map_leader>`_
is a reference to a specific key defined by the mapleader variable and is bound
to backslash by default.

Make sure vim can find :program:`clang-include-fixer`:

- Add the path to :program:`clang-include-fixer` to the PATH environment variable.
- Or set ``g:clang_include_fixer_path`` in vimrc: ``let g:clang_include_fixer_path=path/to/clang-include-fixer``

You can customize the number of headers being shown by setting
``let g:clang_include_fixer_maximum_suggested_headers=5``

Customized settings in `.vimrc`:

- ``let g:clang_include_fixer_path = "clang-include-fixer"``

  Set clang-include-fixer binary file path.

- ``let g:clang_include_fixer_maximum_suggested_headers = 3``

  Set the maximum number of ``#includes`` to show. Default is 3.

- ``let g:clang_include_fixer_increment_num = 5``

  Set the increment number of #includes to show every time when pressing ``m``.
  Default is 5.

- ``let g:clang_include_fixer_jump_to_include = 0``

  Set to 1 if you want to jump to the new inserted ``#include`` line. Default is
  0.

- ``let g:clang_include_fixer_query_mode = 0``

  Set to 1 if you want to insert ``#include`` for the symbol under the cursor.
  Default is 0. Compared to normal mode, this mode won't parse the source file
  and only search the sysmbol from database, which is faster than normal mode.

See ``clang-include-fixer.py`` for more details.

Integrate with Emacs
--------------------
To run `clang-include-fixer` on a potentially unsaved buffer in Emacs.
Ensure that Emacs finds ``clang-include-fixer.el`` by adding the directory
containing the file to the ``load-path`` and requiring the `clang-include-fixer`
in your ``.emacs``:

.. code-block:: console

 (add-to-list 'load-path "path/to/llvm/source/tools/clang/tools/extra/include-fixer/tool/"
 (require 'clang-include-fixer)

Within Emacs the tool can be invoked with the command
``M-x clang-include-fixer``. This will insert the header that defines the
first undefined symbol; if there is more than one header that would define the
symbol, the user is prompted to select one.

To include the header that defines the symbol at point, run
``M-x clang-include-fixer-at-point``.

Make sure Emacs can find :program:`clang-include-fixer`:

- Either add the parent directory of :program:`clang-include-fixer` to the PATH
  environment variable, or customize the Emacs user option
  ``clang-include-fixer-executable`` to point to the file name of the program.

How it Works
============

To get the most information out of Clang at parse time,
:program:`clang-include-fixer` runs in tandem with the parse and receives
callbacks from Clang's semantic analysis. In particular it reuses the existing
support for typo corrections. Whenever Clang tries to correct a potential typo
it emits a callback to the include fixer which then looks for a corresponding
file. At this point rich lookup information is still available, which is not
available in the AST at a later stage.

The identifier that should be typo corrected is then sent to the database, if a
header file is returned it is added as an include directive at the top of the
file.

Currently :program:`clang-include-fixer` only inserts a single include at a
time to avoid getting caught in follow-up errors. If multiple `#include`
additions are desired the program can be rerun until a fix-point is reached.