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

  map ,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 ``,cf`` to
another binding if you need clang-include-fixer on a different key.

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``

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``.

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

- Add the path to :program:`clang-include-fixer` to the PATH environment variable.

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

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.