High level Ruby bindings for Rust

6 releases

0.3.2 May 30, 2022
0.3.1 May 22, 2022
0.2.1 Apr 3, 2022
0.1.0 Feb 26, 2022

#49 in FFI

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

API Docs | GitHub | crates.io

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", Default::default())?;
// 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::Value::check_frozen, or Array#[] becomes magnus::RArray::aref.

Other Ruby methods that are defined only in Ruby must be called with magnus::Value::funcall. All of Magnus' Ruby wrapper types deref to Value, 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::{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", Default::default())?;
    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;

Type Conversions

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

Rust returning / passing values to Ruby

See the magnus::Value type, for all types implementing Into<Value>, 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 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::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::encoding_error("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.

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.

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 = "0.3"

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, Rubygems currently does not support Rust extensions directly, but a Rakefile can be used to compile your Rust extension when the gem is installed.


spec.extensions = ["ext/my_example_gem/Rakefile"]

# actually a build time dependency, but that's not an option.
spec.add_runtime_dependency "rake", "> 1"

See the rust_blank example for an example Rakefile that can be copied into your project without changes. This Rakefile 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"

Compiling Extensions

If you are compiling your extension yourself outside of Rubygems you will need to pass a number of compiler flags as specified by ruby -e'p RbConfig::CONFIG["DLDFLAGS"]'. These may need translating from C compiler args to rustc args. At a minimum the following should work most of the time:

cargo rustc --release -- -C link-arg=-Wl,-undefined,dynamic_lookup

The compiled library will need to be moved from Cargo's target directory into Ruby's load path. On Linux and macOS the library will have the prefix lib added to the extension name, typically you'd want to rename the file to remove this prefix so that you do not need to include it in your Ruby requires. Additionally on macOS the file extension will need to be changed from .dylib to .bundle.

Embedding Ruby in Rust

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


magnus = { version = "0.3", 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);


Magnus contains pre-built bindings for Ruby 2.6 through 3.1 on Linux x86_64, macOS x86_64, macOS aarch64, and Windows x86_64. For other Ruby version/platform combinations bindings will be generated at compile time, this may require libclang to be installed.

The Minimum supported Rust version is currently Rust 1.51.

Support for statically linking Ruby is provided, but not tested.

Support for 32 bit systems is almost certainly broken, patches are welcome.


Magnus can use rb-sys to provide the low-level bindings to Ruby through the rb-sys-interop feature. This also enables the rb_sys module for interoperability with rb-sys.

This can be enabled with:


magnus = { version = "0.3", features = ["rb-sys-interop"] }

This feature should be considered a preview and will be reworked/expanded in future versions.



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.


~52K SLoC