Remacs Talk Transcript

This is a transcript of my Remacs talk at the last Emacs meetup. This is based on my notes, so it may not match exactly what I said on the night.


I’m excited to be here to talk about Remacs!

Remacs is a project to build a Rust version of Emacs. Today, I’m going to talk about what Remacs is, what it isn’t, and our goals.

I will use the terms Remacs or GNU Emacs when I need to be specific, and use the term Emacs as a generic term. Please shout if I’m unclear.

About Me

I’m a Python programmer by day, and I write crazy Emacs things by night. I’ve contributed to many elisp projects, so hopefully I’ve already benefited your Emacs experience in some small way.

I’ve committed patches to dash.el, counsel, magit and I even have commit rights to GNU Emacs. I believe in Emacs as the most powerful practical programmer tool available today.

The Structure of Emacs

Emacs is really just a lisp interpreter called temacs. The Emacs functionality that we know and love is really the standard library that ships with temacs. The temacs binary itself is basically just I/O and GUI glue code on top a bytecode interpreter.

Zooming In

Let’s take a closer look at Emacs’ code structure.

~/src/emacs $ loc
 Language             Files        Lines        Blank      Comment         Code
 Lisp                  1700      1619767       172079       219650      1228038
 C                      282       404244        57153        69636       277455
 Plain Text              31       238671         4587            0       234084
 C/C++ Header           221        85153        10800        17448        56905
 TeX                     28        28245         3983         6425        17837
 Objective-C             11        23744         3662         2377        17705
 Makefile                59        21294         3302         4577        13415
 Autoconf                14         8521         1641         1612         5268

Emacs is largely written in Emacs lisp. That’s great, because elisp is really expressive and any user can extend it. In fact, the core Emacs team is pushing more functionality into elisp over time. For example, undo functionality moved from C to elisp in 2012.

We can see that there’s a lot more elisp than C in Emacs. Since elisp requires fewer lines of code per feature, the vast majority of Emacs functionality is written in elisp.

There’s still a lot of C here. There’s the C code itself, the header files, and build scripts supporting it.

This view is slightly misleading, however. C code doesn’t have a package manager, so there are C libraries in Emacs that have been copy-pasted into the source tree. For example, the MD5 implementation.

This means that some of the C code is less maintained and not all of the functionality at the C level is used anywhere inside Emacs.

Being Emacsy is about providing an incredible mutable programming environment, and things like portably accessing a user’s home directory are not really what Emacs is about.


So, how is Remacs different?

Remacs is an incremental port of that C code to Rust. Since it’s an incremental port, we have a working Emacs instance at every stage of the project.

Note that Remacs is based on the master branch of GNU Emacs, so sometimes we get compatibility bug reports due to GNU Emacs changing something on master.

We aim to be a drop-in replacement with bug-for-bug compatibility.

We’re also seeking to match or beat GNU Emacs performance. We’ve discussed replacing the unexec support, but we’ve been unwilling to move to anything that hurts performance.

I have done a few preliminary benchmarks of Remacs’ primitives, and performance is definitely no worse. It’s possible that it’s slightly faster, but I would need to measure more and be very careful to do a fair comparison.

What Remacs Isn’t

Remacs isn’t a hostile fork. I have commit privileges on GNU Emacs: we haven’t fallen out with the core team.

On the contrary, we have hit bugs in in the test suite (#25534) and interpreter segfaults (#25684) and we’ve reported them upstream. We want to be positive contributors to the Emacs ecosystem.

Remacs is also not a radical overhaul of the Emacs design. From a user’s perspective, they might notice that Remacs is slightly faster, or more robust (although GNU Emacs very rarely crashes). They might also notice that the contribution workflow is different (pull requests on GitHub).

Finally, Remacs is not planning to replace Emacs Lisp. We’ve discussed possible changes to the bytecode format, but Remacs should be a drop-in alternative implementation of emacs lisp.

The C Codebase

GNU Emacs is a large and old C codebase. It has code to support DOS, SunOS, and even some broken malloc implementations.

Remacs will not support these platforms, and that’s OK. GNU Emacs still exists for your DOS computing needs :)

The C codebase predates clang-format, so the code style is not always consistent. The GNU Emacs core team is understandably reluctant to add clang-format as it makes tools like git-blame less effective.

Remacs uses rustfmt for all contributions, because we have that luxury.

Contributing to GNU Emacs follows a traditional workflow: you send your patches to mailing list. There is CI (emacs-trunk on Hydra) but it’s Linux only and after-the-fact (it runs once your changes have been accepted). GNU Emacs’ tests are written in elisp and don’t have unit tests at the C level.

To contribute to Remacs, we use GitHub with pull requests. We use Travis for PRs, so every PR is checked for Rust compiler warnings, we run the Rust unit tests and the whole Emacs test suite on both Linux and OS X.

Why Rust?

Rust is a high performance language that allows you work close to the metal, just like C.

Rust does have a learning curve, but it’s very approachable: the docs and community are both fantastic when you are stuck.

Rust also has a package manager with a growing selection of libraries available. We can pull libraries for things like HTTP and MD5, and focus on what makes Emacs Emacsy. In cases where the Rust ecosystem doesn’t have the libraries we need, we can factor out reusable libraries.

Another big benefit of Rust is that it’s safe. Safe Rust code will not segfault, nor will safe multithreaded code have data races. As Emacs moves towards elisp-level multithreading, Rust can be a huge help here.

A Separate Project

Remacs is a fork that lives on GitHub. This gives us freedom to explore different designs and contribution models. This is much like GuileEmacs, XEmacs and emacs-jit.

We have a lightweight pull request model. There are now four people who can accept PRs, so we’re trying to reduce our bus factor.


Our GitHub repo is now at over 1,000 stars, so there’s definitely interest in our project.

We’ve also been featured on Hacker News, Reddit, Phoronix, Linux Magazin and Golem. This has helped bring new contributors in.


Remacs now has several elisp primitive functions that are written in pure Rust. For example, calling car, cdr, +, + or sin uses Rust implementations.

We’ve also integrated some Rust libraries. If you calculate a SHA256 sum or base64 encode a string, Remacs uses Rust libraries that are maintained by other Rustaceans.

Finally, Remacs passes almost the entire GNU Emacs test suite on both Linux and OS X. (We have a few issues with some EIEIO tests that aren’t deterministic on Travis.) We’ve also developed a modest Rust unit test suite.


There’s now a remacs-git package on the Arch Linux user repository. If you’re an EVM user (a great tool for testing elisp against multiple Emacs versions), Remacs is now available on EVM too!


We now have a low-traffic dedicated Remacs subreddit. We will announce news and milestones here.

We’ve also recently added a Gitter chatroom. If you have questions or want to lurk and see the latest developments, feel free to stop by.


Remacs lives at and we’d love you to try it.

Our README includes a walkthrough for writing your first elisp primitive in Rust. If you’ve ever fancied hacking on Emacs core, we’d love to have you join us.

We’re here to enrich the Emacs ecosystem. Thanks for listening.

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