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Rust :heart: Emacs

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A community-driven port of Emacs to Rust.

Table of Contents

Why Emacs?

Emacs will change how you think about programming.

Emacs is totally introspectable. You can always find out 'what code runs when I press this button?'.

Emacs is an incremental programming environment. There's no edit-compile-run cycle. There isn't even an edit-run cycle. You can execute snippets of code and gradually turn them into a finished project. There's no distinction between your editor and your interpreter.

Emacs is a mutable environment. You can set variables, tweak functions with advice, or redefine entire functions. Nothing is off-limits.

Emacs provides functionality without applications. Rather than separate applications, functionality is all integrated into your Emacs instance. Amazingly, this works. Ever wanted to use the same snippet tool for writing C++ classes as well as emails?

Emacs is full of incredible software concepts that haven't hit the mainstream yet. For example:

  • Many platforms have a single item clipboard. Emacs has an infinite clipboard.
  • If you undo a change, and then continue editing, you can't redo the original change. Emacs allows undoing to any historical state, even allowing tree-based exploration of history.
  • Emacs supports a reverse variable search: you can find variables with a given value.
  • You can perform structural editing of code, allowing you to make changes without breaking syntax. This works for lisps (paredit) and non-lisps (smartparens).
  • Many applications use a modal GUI: for example, you can't do other edits during a find-and-replace operation. Emacs provides recursive editing that allow you to suspend what you're currently doing, perform other edits, then continue the original task.

Emacs has a documentation culture. Emacs includes a usage manual, a lisp programming manual, pervasive docstrings and even an interactive tutorial.

Emacs has a broad ecosystem. If you want to edit code in a niche language, there's probably an Emacs package for it.

Emacs doesn't have a monopoly on good ideas, and there are other great tools out there. Nonetheless, we believe the Emacs learning curve pays off.

Why Rust?

Rust is a great alternative to C.

Rust has a fantastic learning curve. The documentation is superb, and the community is very helpful if you get stuck.

Rust has excellent tooling. The compiler makes great suggestions, the unit test framework is good, and

helps ensure formatting is beautiful and consistent.

The Rust packaging story is excellent. It's easy to reuse the great libraries available, and just as easy to factor out code for the benefit of others. We can replace entire C files in Emacs with well-maintained Rust libraries.

Code written in Rust easily interoperates with C. This means we can port to Rust incrementally, and having a working Emacs at each step of the process.

Rust provides many compile-time checks, making it much easier to write fast, correct code (even when using multithreading). This also makes it much easier for newcomers to contribute.

Give it a try. We think you'll like it.

Why A Fork?

Emacs is a widely used tool with a long history, broad platform support and strong backward compatibility requirements. The core team is understandably cautious in making far-reaching changes.

Forking is a longstanding tradition in the Emacs community for trying different approaches. Notable Emacs forks include XEmacs, Guile Emacs, and emacs-jit.

There have also been separate elisp implementations, such as Deuce, JEmacs and El Compilador.

By forking, we can explore new development approaches. We can use a pull request workflow with integrated CI.

We can drop legacy platforms and compilers. Remacs will never run on MS-DOS, and that's OK.

There's a difference between the idea of Emacs and the current implementation of Emacs. Forking allows us to explore being even more Emacs-y.

Getting Started


  1. You will need Rust installed. The file

    indicates the version that gets installed. This happens automatically, so don't override the toolchain manually. IMPORTANT: Whenever the toolchain updates, you have to reinstall rustfmt manually.
  2. You will need a C compiler and toolchain. On Linux, you can do something like:

    apt install build-essential automake clang libclang-dev

On macOS, you'll need Xcode.

  1. Linux:

    apt install texinfo libjpeg-dev libtiff-dev \
      libgif-dev libxpm-dev libgtk-3-dev gnutls-dev \
      libncurses5-dev libxml2-dev libxt-dev


    brew install gnutls texinfo autoconf

To use the installed version of makeinfo instead of the built-in (/usr/bin/makeinfo) one, you'll need to make sure /usr/local/opt/texinfo/bin is before /usr/bin in PATH. Mojave install libxml2 headers with: open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

Dockerized development environment

If you don't want to bother with the above setup you can use the provided Docker environment. Make sure you have docker 1.12+ and docker-compose 1.8+ available.

To spin up the environment run

docker-compose up -d

The first time you run this command, Docker will build the image. After that any subsequent startups will happen in less than a second. If this command fails because of needing absolute paths, make sure to set the PWD environment variable before calling the command like so:

PWD=$(pwd) docker-compose up -d

The working directory with remacs will be mounted under the same path in the container so editing the files on your host machine will automatically be reflected inside the container. To build remacs use the steps from Building Remacs prefixed with

docker-compose exec remacs
, this will ensure the commands are executed inside the container.

Building Remacs

$ ./autogen.sh
$ ./configure --enable-rust-debug
$ make

For a release build, don't pass


The Makefile obeys cargo's RUSTFLAGS variable and additional options can be passed to cargo with CARGO_FLAGS.

For example:

$ make CARGO_FLAGS="-vv" RUSTFLAGS="-Zunstable-options --cfg MARKER_DEBUG"

Running Remacs

You can now run your shiny new Remacs build!

# Using -q to ignore your .emacs.d, so Remacs starts up quickly.
# RUST_BACKTRACE is optional, but useful if your instance crashes.
$ RUST_BACKTRACE=1 src/remacs -q

Design Goals

Compatibility: Remacs should not break existing elisp code, and ideally provide the same FFI too.

Leverage Rust itself: Remacs should make best use of Rust to ensure code is robust and performant.

Leverage the Rust ecosystem: Remacs should use existing Rust crates wherever possible, and create new, separate crates where our code could benefit others.

Great docs: Emacs has excellent documentation, Remacs should be no different.


At this point we focus on porting lisp functions from C to Rust. Currently there are 642 functions in Rust and 823 in C (May 2019).

We have a progress section in our wiki and there's also a list of long-term goals under projects.

Porting Elisp Primitive Functions

The first thing to look at is the C implementation for the

function. It takes an optional second argument, which makes it interesting. The complicated mathematical bits, on the other hand, are handled by the standard library. This allows us to focus on the porting process without getting distracted by the math.

The Lisp values we are given as arguments are tagged pointers; in this case they are pointers to doubles. The code has to check the tag and follow the pointer to retrieve the real values. Note that this code invokes a C macro (called

) that reduces some of the boilerplate. The macro declares a static variable called
that holds the metadata the Lisp compiler will need in order to successfully call this function, such as the docstring and the pointer to the
function, which is what the C implementation is named:
DEFUN ("atan", Fatan, Satan, 1, 2, 0,
       doc: /* Return the inverse tangent of the arguments.
If only one argument Y is given, return the inverse tangent of Y.
If two arguments Y and X are given, return the inverse tangent of Y
divided by X, i.e. the angle in radians between the vector (X, Y)
and the x-axis.  */)
  (Lisp_Object y, Lisp_Object x)
  double d = extract_float (y);
  if (NILP (x))
    d = atan (d);
      double d2 = extract_float (x);
      d = atan2 (d, d2);
  return make_float (d);

checks the tag (signalling an "invalid argument" error if it's not the tag for a double), and returns the actual value.
checks to see if the tag indicates that this is a null value, indicating that the user didn't supply a second argument at all.

Next take a look at the current Rust implementation. It must also take an optional argument, and it also invokes a (Rust) macro to reduce the boilerplate of declaring the static data for the function. However, it also takes care of all of the type conversions and checks that we need to do in order to handle the arguments and return value:

/// Return the inverse tangent of the arguments.
/// If only one argument Y is given, return the inverse tangent of Y.
/// If two arguments Y and X are given, return the inverse tangent of Y
/// divided by X, i.e. the angle in radians between the vector (X, Y)
/// and the x-axis
#[lisp_fn(min = "1")]
pub fn atan(y: EmacsDouble, x: Option) -> EmacsDouble {
    match x {
        None => y.atan(),
        Some(x) => y.atan2(x)

You can see that we don't have to check to see if our arguments are of the correct type, the code generated by the

macro does this for us. We also asked for the second argument to be an
. This is the Rust type for a value which is either a valid double or isn't specified at all. We use a match statement to handle both cases.

This code is so much better that it's hard to believe just how simple the implementation of the macro is. It just calls

on the arguments and the return value; the compiler does the rest when it dispatches this method call to the correct implementation.


Pull requests welcome, no copyright assignment required. This project is under the Rust code of conduct.

There's lots to do! We keep a list of low hanging fruit here so you can easily choose one. You can find information in the Porting cookbook or ask for help in our Gitter channel.

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