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High level Ruby bindings. Write Ruby extension gems in Rust, or call Ruby code from a Rust binary.

21 releases

0.6.3 Apr 1, 2024
0.6.2 Sep 19, 2023
0.6.1 Aug 20, 2023
0.6.0 Jul 29, 2023
0.1.0 Feb 26, 2022

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High level Ruby bindings for Rust. Write Ruby extension gems in Rust, or call Ruby code from a Rust binary.

API Docs | GitHub | crates.io

Getting Started | Type Conversions | Safety | Compatibility


Defining Methods

Using Magnus, regular Rust functions can be bound to Ruby as methods with automatic type conversion. Callers passing the wrong arguments or incompatible types will get the same kind of ArgumentError or TypeError they are used to seeing from Ruby's built in methods.

Defining a function (with no Ruby self argument):

fn fib(n: usize) -> usize {
    match n {
        0 => 0,
        1 | 2 => 1,
        _ => fib(n - 1) + fib(n - 2),

magnus::define_global_function("fib", magnus::function!(fib, 1));

Defining a method (with a Ruby self argument):

fn is_blank(rb_self: String) -> bool {
    !rb_self.contains(|c: char| !c.is_whitespace())

let class = magnus::define_class("String", magnus::class::object())?;
// 0 as self doesn't count against the number of arguments
class.define_method("blank?", magnus::method!(is_blank, 0))?;

Calling Ruby Methods

Some Ruby methods have direct counterparts in Ruby's C API and therefore in Magnus. Ruby's Object#frozen? method is available as magnus::ReprValue::check_frozen, or Array#[] becomes magnus::RArray::aref.

Other Ruby methods that are defined only in Ruby must be called with magnus::ReprValue::funcall. All of Magnus' Ruby wrapper types implement the ReprValue trait, so funcall can be used on all of them.

let s: String = value.funcall("test", ())?; // 0 arguments
let x: bool = value.funcall("example", ("foo",))?; // 1 argument
let i: i64 = value.funcall("other", (42, false))?; // 2 arguments, etc

funcall will convert return types, returning Err(magnus::Error) if the type conversion fails or the method call raised an error. To skip type conversion make sure the return type is magnus::Value.

Wrapping Rust Types in Ruby Objects

Rust structs and enums can be wrapped in Ruby objects so they can be returned to Ruby.

Types can opt-in to this with the magnus::wrap macro (or by implementing magnus::TypedData). Whenever a compatible type is returned to Ruby it will be wrapped in the specified class, and whenever it is passed back to Rust it will be unwrapped to a reference.

use magnus::{class, define_class, function, method, prelude::*, Error};

#[magnus::wrap(class = "Point")]
struct Point {
    x: isize,
    y: isize,

impl Point {
    fn new(x: isize, y: isize) -> Self {
        Self { x, y }

    fn x(&self) -> isize {

    fn y(&self) -> isize {

    fn distance(&self, other: &Point) -> f64 {
        (((other.x - self.x).pow(2) + (other.y - self.y).pow(2)) as f64).sqrt()

fn init() -> Result<(), Error> {
    let class = define_class("Point", class::object())?;
    class.define_singleton_method("new", function!(Point::new, 2))?;
    class.define_method("x", method!(Point::x, 0))?;
    class.define_method("y", method!(Point::y, 0))?;
    class.define_method("distance", method!(Point::distance, 1))?;

The newtype pattern and RefCell can be used if mutability is required:

struct Point {
    x: isize,
    y: isize,

#[magnus::wrap(class = "Point")]
struct MutPoint(std::cell::RefCell<Point>);

impl MutPoint {
    fn set_x(&self, i: isize) {
        self.0.borrow_mut().x = i;

Getting Started

Writing an extension gem (calling Rust from Ruby)

Ruby extensions must be built as dynamic system libraries, this can be done by setting the crate-type attribute in your Cargo.toml.


crate-type = ["cdylib"]

magnus = "0.6"

When Ruby loads your extension it calls an 'init' function defined in your extension. In this function you will need to define your Ruby classes and bind Rust functions to Ruby methods. Use the #[magnus::init] attribute to mark your init function so it can be correctly exposed to Ruby.


use magnus::{define_global_function, function};

fn distance(a: (f64, f64), b: (f64, f64)) -> f64 {
    ((b.0 - a.0).powi(2) + (b.1 - a.1).powi(2)).sqrt()

fn init() {
    define_global_function("distance", function!(distance, 2));

If you wish to package your extension as a Gem, we recommend using the rb_sys gem to build along with rake-compiler. These tools will automatically build your Rust extension as a dynamic library, and then package it as a gem.

Note: The newest version of rubygems does have beta support for compiling Rust, so in the future the rb_sys gem won't be necessary.


spec.extensions = ["ext/my_example_gem/extconf.rb"]

# needed until rubygems supports Rust support is out of beta
spec.add_dependency "rb_sys", "~> 0.9.39"

# only needed when developing or packaging your gem
spec.add_development_dependency "rake-compiler", "~> 1.2.0"

Then, we add an extconf.rb file to the ext directory. Ruby will execute this file during the compilation process, and it will generate a Makefile in the ext directory. See the rb_sys gem for more information.


require "mkmf"
require "rb_sys/mkmf"


See the rust_blank example for examples if extconf.rb and Rakefile. Running rake compile will place the extension at lib/my_example_gem/my_example_gem.so (or .bundle on macOS), which you'd load from Ruby like so:


require_relative "my_example_gem/my_example_gem"

For a more detailed example (including cross-compilation and more), see the rb-sys example project. Although the code in lib.rs does not feature magnus, but it will compile and run properly.

Embedding Ruby in Rust

To call Ruby from a Rust program, enable the embed feature:


magnus = { version = "0.6", features = ["embed"] }

This enables linking to Ruby and gives access to the embed module. magnus::embed::init must be called before calling Ruby and the value it returns must not be dropped until you are done with Ruby. init can not be called more than once.


use magnus::{embed, eval};

fn main() {
    let _cleanup = unsafe { embed::init() };

    let val: f64 = eval!("a + rand", a = 1).unwrap();

    println!("{}", val);

Type Conversions

Magnus will automatically convert between Rust and Ruby types, including converting Ruby exceptions to Rust Results and vice versa.

These conversions follow the pattern set by Ruby's core and standard libraries, where many conversions will delegate to a #to_<type> method if the object is not of the requested type, but does implement the #to_<type> method.

Below are tables outlining many common conversions. See the Magnus api documentation for the full list of types.

Rust functions accepting values from Ruby

See magnus::TryConvert for more details.

Rust function argument accepted from Ruby
i8,i16,i32,i64,isize, magnus::Integer Integer, #to_int
u8,u16,u32,u64,usize Integer, #to_int
f32,f64, magnus::Float Float, Numeric
String, PathBuf, char, magnus::RString, bytes::Bytes*** String, #to_str
magnus::Symbol Symbol, #to_sym
bool any object
magnus::Range Range
magnus::Encoding, magnus::RbEncoding Encoding, encoding name as a string
Option<T> T or nil
(T, U), (T, U, V), etc [T, U], [T, U, V], etc, #to_ary
[T; N] [T], #to_ary
magnus::RArray Array, #to_ary
magnus::RHash Hash, #to_hash
magnus::Value any object
Vec<T>* [T], #to_ary
HashMap<K, V>* {K => V}, #to_hash
&T, typed_data::Obj<T> where T: TypedData** instance of <T as TypedData>::class()

* when converting to Vec and HashMap the types of T/K,V must be native Rust types.

** see the wrap macro.

*** when the bytes feature is enabled

Rust returning / passing values to Ruby

See magnus::IntoValue for more details, plus magnus::method::ReturnValue and magnus::ArgList for some additional details.

returned from Rust / calling Ruby from Rust received in Ruby
i8,i16,i32,i64,isize Integer
u8,u16,u32,u64,usize Integer
f32, f64 Float
String, &str, char, &Path, PathBuf String
bool true/false
() nil
Range, RangeFrom, RangeTo, RangeInclusive Range
Option<T> T or nil
Result<T, magnus::Error> (return only) T or raises error
(T, U), (T, U, V), etc, [T; N], Vec<T> Array
HashMap<K, V> Hash
T, typed_data::Obj<T> where T: TypedData** instance of <T as TypedData>::class()

** see the wrap macro.

Manual Conversions

There may be cases where you want to bypass the automatic type conversions, to do this use the type magnus::Value and then manually convert or type check from there.

For example, if you wanted to ensure your function is always passed a UTF-8 encoded String so you can take a reference without allocating you could do the following:

fn example(val: magnus::Value) -> Result<(), magnus::Error> {
    // checks value is a String, does not call #to_str
    let r_string = RString::from_value(val)
        .ok_or_else(|| magnus::Error::new(magnus::exception::type_error(), "expected string"))?;
    // error on encodings that would otherwise need converting to utf-8
    if !r_string.is_utf8_compatible_encoding() {
        return Err(magnus::Error::new(
            "string must be utf-8",
    // RString::as_str is unsafe as it's possible for Ruby to invalidate the
    // str as we hold a reference to it. The easiest way to ensure the &str
    // stays valid is to avoid any other calls to Ruby for the life of the
    // reference (the rest of the unsafe block).
    unsafe {
        let s = r_string.as_str()?;
        // ...


When using Magnus, in Rust code, Ruby objects must be kept on the stack. If objects are moved to the heap the Ruby GC can not reach them, and they may be garbage collected. This could lead to memory safety issues.

It is not possible to enforce this rule in Rust's type system or via the borrow checker, users of Magnus must maintain this rule manually.

An example of something that breaks this rule would be storing a Ruby object in a Rust heap allocated data structure, such as Vec, HashMap, or Box. This must be avoided at all costs.

While it would be possible to mark any functions that could expose this unsafty as unsafe, that would mean that almost every interaction with Ruby would be unsafe. This would leave no way to differentiate the really unsafe functions that need much more care to use.

Other than this, Magnus strives to match Rust's usual safety guaranties for users of the library. Magnus itself contains a large amount of code marked with the unsafe keyword, it is impossible to interact with Ruby's C-api without this, but users of Magnus should be able to do most things without needing to use unsafe.


Ruby versions 3.0, 3.1, and 3.2 are fully supported.

Magnus currently works with, and is still tested against, Ruby 2.7, but as this version of the language is no longer supported by the Ruby developers it is not recommended and future support in Magnus is not guaranteed.

Magnus is no longer tested against Ruby 2.6. Code supporting 2.6 has not been removed, but there is no guarantee it will continue to work. Minor patches for Ruby 2.6 compatibility will be accepted.

Ruby bindings will be generated at compile time, this may require libclang to be installed.

The Minimum supported Rust version is currently Rust 1.61.

Support for statically linking Ruby is provided.

Cross-compilation is supported by rb-sys for the platforms listed here.

Magnus is not tested on 32 bit systems. Efforts are made to ensure it compiles. Patches are welcome.

Crates that work with Magnus


Magnus uses rb-sys to provide the low-level bindings to Ruby. The rb-sys feature enables the rb_sys module for advanced interoperability with rb-sys, allows you to access low-level Ruby APIs which Magnus does not expose.


serde_magnus integrates [Serde] and Magnus for seamless serialisation and deserialisation of Rust to Ruby data structures and vice versa.


  • halton a Ruby gem providing a highly optimised method for generating Halton sequences.

Please open a pull request if you'd like your project listed here.


Issues with static linking

If you encounter an error such as symbol not found in flat namespace '_rb_ext_ractor_safe' when embedding static Ruby, you will need to instruct Cargo not to strip code that it thinks is dead.

In you the same directory as your Cargo.toml file, create a .cargo/config.toml file with the following contents:

# Without this flag, when linking static libruby, the linker removes symbols
# (such as `_rb_ext_ractor_safe`) which it thinks are dead code... but they are
# not, and they need to be included for the `embed` feature to work with static
# Ruby.
rustflags = ["-C", "link-dead-code=on"]


Magnus is named after Magnus the Red a character from the Warhammer 40,000 universe. A sorcerer who believed he could tame the psychic energy of the Warp. Ultimately, his hubris lead to his fall to Chaos, but lets hope using this library turns out better for you.


This project is licensed under the MIT license, see LICENSE.


~55K SLoC