Rust ❤️ Emacs
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Rust ❤️ Emacs

Build Status

A community-driven port of Emacs to Rust.

GPLv3 license.

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 rustfmt 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.

Design Goals

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

Similar naming conventions: Code in Remacs should use the same naming conventions for elisp namespaces, to make translation straightforward.

This means that an elisp function do-stuff will have a corresponding Rust function Fdo_stuff, and a declaration struct Sdo_stuff. A lisp variable do-stuff will have a Rust variable Vdo_stuff and a symbol 'do-stuff will have a Rust variable Qdo_stuff.

Otherwise, we follow Rust naming conventions, with docstrings noting equivalent functions or macros in C. When incrementally porting, we may define Rust functions with the same name as their C predecessors.

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.

Non-Design Goals

etags: The universal ctags project supports a wider range of languages and we recommend it instead.

Building Remacs

$ ./
$ ./configure
$ make

You can then run your shiny new Remacs:

# 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

Debug builds

As above but with an additional argument to configure:

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

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

For example:

$ make CARGO_FLAGS="-vv" RUSTFLAGS="-Zunstable-options --pretty"

Rustdoc builds

You can use rustdoc to generate API docs:

$ cargo rustdoc -- \
    --no-defaults \
    --passes strip-hidden \
    --passes collapse-docs \
    --passes unindent-comments \
    --passes strip-priv-imports

You can then open these docs with:

$ cargo doc --open

Porting Elisp Primitive Functions: Walkthrough

Let's look at porting numberp to Rust.

First, make sure you have configured and built Remacs on your system. You'll probably want to generate TAGS too, so you can jump to definitions of C functions.

Emacs C uses a lot of macros, so it's also useful to look at the expanded version of the code.

Define a little file src/dummy.c with the C source of numberp, along with the lisp.h header file:

#include "lisp.h"

DEFUN ("numberp", Fnumberp, Snumberp, 1, 1, 0,
       doc: /* Return t if OBJECT is a number (floating point or integer).  */
       attributes: const)
  (Lisp_Object object)
  if (NUMBERP (object))
    return Qt;
    return Qnil;

Then expand it with GCC:

$ cd /path/to/remacs
$ gcc -Ilib -E src/dummy.c > dummy_exp.c

This gives us a file that ends with:

static struct Lisp_Subr 
# 3 "src/dummy.c" 3 4
# 3 "src/dummy.c"
(8) Snumberp = { { PVEC_SUBR << PSEUDOVECTOR_AREA_BITS }, { .a1 = Fnumberp }, 1, 1, "numberp", 0, 0}; Lisp_Object Fnumberp

  (Lisp_Object object)
  if (NUMBERP (object))
    return Qt;
    return builtin_lisp_symbol (0);

We can see we need to define a Snumberp and a Fnumberp. Qt and Qnil are already defined in, so we can simply write:

// This is the function that gets called when 
// we call numberp in elisp.
fn Fnumberp(object: LispObject) -> LispObject {
    if lisp::NUMBERP(object) {
        unsafe {
    } else {

// This defines a built-in function in elisp, which is a represented
// with a static struct.
defun!("numberp", // the name of our elisp function
       Fnumberp, // the rust function we want to call
       Snumberp, // the name of the struct that we will define
       1, 1, // min and max number of arguments
       ptr::null(), // our function is not interactive
       // docstring, the last line ensures that *Help* shows the
       // correct calling convention
       "Return t if OBJECT is a number (floating point or integer).

(fn OBJECT)");

Finally, we need to delete the old C definition and call defsubr inside rust_init_syms:

pub extern "C" fn rust_init_syms() {
    unsafe {
        // ...

You're done! Compile Remacs, try your function with M-x ielm, and open a pull request. Fame and glory await!

Porting Widely Used C Functions

If your Rust function replaces a C function that is used elsewhere in the C codebase, you will need to export it. We change our function definition to add extern and no_mangle:

pub extern "C" fn Fnumberp(object: LispObject) -> LispObject {
    if lisp::NUMBERP(object) {
        unsafe {
    } else {

The function needs to be exported in

pub use yourmodulename::Fnumberp;

and add a declaration in the C where the function used to be:

// This should take the same number of arguments as the Rust function.
Lisp_Object Fnumberp(Lisp_Object);


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

Help Needed

There's lots to do! We keep a list of low hanging fruit here so you can easily choose one. If you do, please open a new issue to keep track of the task and link to it.

Easy tasks:

  • Find a small function in lisp.h and write an equivalent in
  • Improve our unit tests. Currently we're passing Qnil to test functions, which isn't very useful.
  • Add docstrings to public functions in
  • Tidy up messy Rust that's been translated directly from C. Run rustfmt, add or rename internal variables, run clippy, and so on.
  • Fix the makefile to recompile with cargo and rebuild temacs when the Rust source changes.
  • Add Rust-level unit tests to elisp functions defined in

Medium tasks:

  • Choose an elisp function you like, and port it to rust. Look at rust-mod for an example.
  • Teach describe-function to find functions defined in Rust.
  • Expand our Travis configuration to run 'make check', so we know remacs passes Emacs' internal test suite.
  • Expand our Travis configuration to ensure that Rust code has been formatted with rustfmt
  • Set up bors/homu.
  • Set up a badge tracking pub struct/function coverage using cargo-doc-coverage.
  • Search the Rust source code for TODO comments and fix them.
  • Teach Emacs how to jump to definition for Rust functions.

Big tasks:

  • Find equivalent Rust libraries for parts of Emacs, and replace all the relevant C code. Rust has great libraries for regular expressions, GUI, terminal UI, managing processes, amongst others.
  • Change the elisp float representation to use nan boxing rather than allocating floats on the heap.

Rust Porting Tips

C Functions

When writing a Rust version of a C function, give it the same name and same arguments. If this isn't appropriate, docstrings should say the equivalent C function to help future porters.

For example, make_natnum mentions that it can be used in place of XSETFASTINT.

C Macros

For C macros, we try to define a fairly equivalent Rust function. The docstring should mention the original macro name.

Since the Rust function is not a drop-in replacement, we prefer Rust naming conventions for the new function.

For the checked arithmetic macros (INT_ADD_WRAPV, INT_MULTIPLY_WRAPV and so on), you can simply use .checked_add, .checked_mul from the Rust stdlib.


eassert in Emacs C should be debug_assert! in Rust.

emacs_abort() in Emacs C should be panic!("reason for panicking") in Rust.


LispObject values may represent pointers, so the usual safety concerns of raw pointers apply.

If you can break memory safety by passing a valid value to a function, then it should be marked as unsafe. For example:

// This function is unsafe because it's dereferencing the car
// of a cons cell. If `object` is not a cons cell, we'll dereference
// an invalid pointer.
unsafe fn XCAR(object: LispObject) -> LispObject {

// This function is safe because it preserves the contract
// of XCAR: it only passes valid cons cells. We just use
// unsafe blocks instead.
fn car(object: LispObject) -> LispObject {
    if CONSP(object) {
        unsafe {
    } else if NILP(object) {
    } else {
        unsafe {
            wrong_type_argument(Qlistp, object)