Ruby bindings for Rust. Write Ruby extension gems in Rust, or call Ruby code from a Rust binary.
Getting Started | Type Conversions | Safety | Compatibility
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))?;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, etcfuncall 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.
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 {
self.x
}
fn y(&self) -> isize {
self.y
}
fn distance(&self, other: &Point) -> f64 {
(((other.x - self.x).pow(2) + (other.y - self.y).pow(2)) as f64).sqrt()
}
}
#[magnus::init]
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))?;
Ok(())
}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;
}
}Ruby extensions must be built as dynamic system libraries, this can be done by
setting the crate-type attribute in your Cargo.toml.
Cargo.toml
[lib]
crate-type = ["cdylib"]
[dependencies]
magnus = "0.4"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.
src/lib.rs
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()
}
#[magnus::init]
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.
my_example_gem.gemspec
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.
ext/my_example_gem/extconf.rb
require "mkmf"
require "rb_sys/mkmf"
create_rust_makefile("my_example_gem/my_example_gem")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:
lib/my_example_gem.rb
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.
To call Ruby from a Rust program, enable the embed feature:
Cargo.toml
[dependencies]
magnus = { version = "0.4", 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.
src/main.rs
use magnus::{embed, eval};
fn main() {
let _cleanup = unsafe { embed::init() };
let val: f64 = eval!("a + rand", a = 1).unwrap();
println!("{}", val);
}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.
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.
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.
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()?;
// ...
}
Ok(())
}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.
Ruby versions 2.7, 3.0, and 3.1 are fully supported.
Ruby 3.2 is (as of writing) in preview and support is provided, but not guaranteed.
Magnus currently works with Ruby 2.6, 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.
Ruby bindings will be generated at compile time, this may require libclang to be installed.
The Minimum supported Rust version is currently Rust 1.51 (Rust 1.54 on macOS with Xcode 14).
Support for statically linking Ruby is provided.
Cross-compilation is supported by rb-sys for the platforms listed here.
Support for 32 bit systems is almost certainly broken, patches are welcome.
Magnus uses rb-sys to provide the low-level bindings to Ruby. The
rb-sys-interop 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.
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:
[build]
# 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.