See the devtools documentation that describes how to integrate ESLint into Emacs.

C/C++ Development Packages

General Guidelines to Emacs C++ Programming

The following guides give an overview of the C++ editing capabilities of emacs.

It is worth reading through these guides to see what features are available. The rest of this section is dedicated to Mozilla/Gecko specific setup for packages.

rtags (LLVM/Clang-based Code Indexing)

Instructions for the installation of rtags are available at the rtags github repo.

rtags requires a compilation database.

In order for rtags to index correctly, included files need to be copied and unified compilation files need to be created. Either run a full build of the tree, or if you only want indexes to be generated for the moment, run the following commands (assuming you’re in the gecko repo root):

cd gecko_build_directory
make export

To increase indexing speed, it’s best to remove unified build files and test files from database updates. This can be done by creating a ~/.rdmrc file with the following contents, with [src_dir] replaced with either the repo or build directory for your checkout:

-X */[src_dir]/*Unified*;*/[src_dir]/*/test/*;*/[src_dir]/*/tests/*

Once the rdm daemon is running, the compilation database can be added to rtags like so:

rc -J [gecko_build_directory]/compile_commands.json

Note that this process will take a while initially. However, once the database is built, it will only require incremental updates. As long as the rdm daemon is running in the background, the database will be updated based on changes to files.

irony (LLVM/Clang-based Code Completion)

Instructions on the installation of irony-mode are available at the irony-mode github repo.

irony-mode requires a compilation database.

Note that irony-mode, by default, uses elisp to parse the compile_commands.json file. As gecko is a very large codebase, this file can easily be multiple megabytes, which can make irony-mode take multiple seconds to load on a gecko file.

It is recommended to use this fork of irony-mode, which requires the boost System and Filesystem libraries.

Checking the bug to get this patch into the mainline of irony-mode is recommended, to see if the fork can be used or if the mainline repo can be used. Using the Boost version of the irony-mode server brings file load times to under 1s.

Projectile (Project Management)

Instructions on the installation of projectile are available at the projectile github repo.

Projectile comes preconfigured for many project types. Since, gecko uses its own special build system (mach), a new project type needs to be added. This can be done via adding the following elisp configuration command to your emacs configuration file.

(projectile-register-project-type 'gecko
                                  '("mach" "moz.build")
                                  "python mach --log-no-times build"
                                  "python mach mochitest"
                                  "python mach run")

Assuming projectile-global-mode is on, this will allow projectile to run the correct commands whenever it is working in a gecko repo.


Emacs comes with great integration with gdb, especially when using gdb-many-windows.

However, when gdb is invoked via mach, some special arguments need to be passed in order to make sure the correct display mode is used. To use M-x gdb with mach on firefox, use the following command:

gecko_repo_directory/mach run --debug --debugparams=-i=mi