10 releases

0.0.9 May 29, 2019
0.0.8 May 23, 2019

#48 in #ruby

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High-level, zero (or low) cost bindings of Ruby's C API for Rust.



  • Performance:

    Rosy enables you to write the most performant code possible, such that using the C API directly would not improve performance. In other words, it presents zero-cost abstractions. However, not all of Rosy's safe abstractions are zero-cost. Sometimes this is only possible by writing some unsafe code since Rosy can't be made aware of certain aspects of the program state.

    • For example, Object::call will catch any raised Ruby exceptions via the protected family of functions. On the other hand, Object::call_unchecked will allow any thrown exception propagate (which causes a segmentation fault in Rust-land) unless protected.

      Checking for exceptions via protected has a cost associated with it. It is best to wrap multiple instances of unchecked exception-throwing functions. This allows for reducing the number of speed bumps in your code.

    • If it is known that no panic! will occur anywhere within exception-checked code, then calling protected_no_panic will emit fewer instructions at the cost of safety. The FnOnce passed into this function is called within an FFI context; because of that, panicking here is undefined behavior. Panics in a normal protected call are safely caught with the stack unwinding properly.

    Note that unsafe functions suffixed with _unchecked always have a safe counterpart. Before reaching for unsafe functions, consider using these instead and profiling your code to find out whether it's actually necessary.

  • Powerful Types:

    Rosy leverages Rust's type system to the fullest.

  • Safety: *where possible

    Rosy exposes safe abstractions over most of Ruby's C API. Wherever this isn't possible, such functionality is marked as unsafe with a documented explanation on safe usage.

    Unfortunately, due to the inherent nature of Ruby's C API, safety is often not easily achievable without a few compromises.

    • Many Ruby functions can raise exceptions, which trigger a segmentation fault in Rust-land. 😓

      Functions that may raise an exception are marked as unsafe or have a safe exception-checking equivalent via protected. However, checking for an exception has a cost in performance.

    • Ruby's garbage collector de-deallocates objects whose references don't live on the stack, unless they are marked. This may lead to a possible use after free. When wrapping Rust data, it is important to implement Rosy::mark correctly.


Rosy requires cargo and an existing Ruby installation:

  • cargo can be installed via rustup

  • Ruby can be installed rvm, rbenv, or your favorite package manager

The rosy crate is available on crates.io and can be used by adding the following to your project's Cargo.toml:

rosy = "0.0.9"

Rosy has functionality that is only available for certain Ruby versions. The following features can currently be enabled:

  • ruby_2_6

For example:

version = "0.0.9"
features = ["ruby_2_6"]

Finally add this to your crate root (main.rs or lib.rs):

extern crate rosy;


Rosy can be compiled by simply running:

cargo build

It will automatically try to find the dynamic library via the current ruby installation.

To enable static linking, specify the static feature flag:

version = "0.0.9"
features = ["static"]

To use a specific Ruby installation, you can do either of the following:

  • Set ROSY_RUBY=path/to/ruby

    This must point to an executable.

  • Set ROSY_RUBY=client:version. For example:

    • ROSY_RUBY=rvm:2.6.0

    • ROSY_RUBY=rbenv:2.5.0

    If the :version portion is not provided, then ROSY_RUBY_VERSION is used to get the version number. For example:

    ROSY_RUBY=rvm ROSY_RUBY_VERSION=2.6.0 cargo build


Rosy allows you to perform many operations over Ruby objects in a way that feels very natural in Rust.

Managing Ruby's Virtual Machine

The virtual machine must be initialized via vm::init before doing anything:

rosy::vm::init().expect("Could not initialize Ruby");

Once finished with Ruby, you can clean up its resources permanently via vm::destroy:

if let Err(code) = unsafe { rosy::vm::destroy() } {

Note that you can no longer use Ruby once the VM has been destroyed.

Calling Ruby Methods

Using Object::call, any method can be invoked on the receiving object:

use rosy::String;

let string = String::from("hello\r\n");

assert_eq!(string, "hello");

To pass in arguments, use Object::call_with:

use rosy::{Array, Integer, Object};

let array: Array<Integer> = (10..20).collect();

let args: &[Integer] = &[1.into(), 5.into(), 9.into()];

let values = array.call_with("values_at", args).unwrap();

assert_eq!(values, [11, 15, 19][..]);

Defining Ruby Methods

To define a UTF-8-aware method blank? on Ruby's String class, one can very simply use the def_method! macro. This allows for defining a function that takes the typed object (in this case String) for the class as its receiver.

use rosy::prelude::*;

let class = Class::of::<String>();

rosy::def_method!(class, "blank?", |this: String| {

let string = String::from(" \r\n");
let result = string.call("blank?");

assert_eq!(result.unwrap(), true);

Although the macro may feel somewhat magical, it's actually just a zero-cost wrapper around Class::def_method, which itself is a low-cost abstraction over rb_define_method_id. To bring the abstraction cost down to absolute zero, use def_method_unchecked!.

Defining Ruby Classes

Defining a new class is rather straightforward:

let my_object = Class::def("MyObject").unwrap();

Attempting to define an existing class will result in an error:

let array = Class::def("Array")

assert_eq!(array, Class::array());

To get an existing named class if it's not a built-in class, one should call Class::get:

let my_object = Class::get("MyObject").unwrap();

And if it's ambiguous as to whether the class already exists, there's the best of both worlds: Class::get_or_def. This will define a class with the given name if it doesn't already exist.

let my_object = Class::get_or_def("MyObject").unwrap();

To define a class within the namespace of a class or module, use Mixin::def_class.

Defining Ruby Subclasses

The Class::subclass method allows for creating a new class that inherits from the method receiver's class.

let sub_object = my_object.subclass("MyObjectChild").unwrap();

assert!(sub_object < my_object);

To define a subclass within the namespace of a class or module, use Mixin::def_subclass.

Catching Ruby Exceptions

Rust code can be protected from Ruby exceptions very easily.

use rosy::{Object, String, protected};

let string = String::from("¡Hola!");

let result = protected(|| unsafe {


Platform Support

Rosy uses aloxide to find and link Ruby during its build phase. Because of that, Rosy's platform support is totally dependent on it. Changes that fix issues with linking (or in the future, building) Ruby should be submitted to that library for use in this one.

To work locally on aloxide and Rosy in combination with each other, change Rosy's Cargo.toml like so:

aloxide = { path = "path/to/aloxide", version = "0.0.8", default-features = false }

Library Comparison

Like with most technologies, Rosy isn't the first of its kind.

Rosy vs Helix

Helix is a Rust library built on top of macros. Interaction with the Ruby runtime is done via a ruby! macro which features a DSL that's a mix between Rust and Ruby syntax. To those coming from Ruby, they'll feel right at home. However, those coming from Rust may feel that the macro is a little too magical.

Unlike Helix, for each of Rosy's macros, there's an alternative approach that can be taken purely through types, traits, and functions. Rosy is designed to be convenient and high-level while trying not to hide the low-level details that can allow you to write better-optimized code. This is parallel to the way that Rust acts as a high-level language.

Rosy vs Rutie

Rutie is a Rust library that tries to be less magical than Helix. It is a continuation of the work done on ruru, which is no longer maintained as of the end of 2017. Rutie takes an excellent approach to wrapping Ruby's C API in Rust by exposing Ruby classes as Rust structs. This inspired the layout and design of Rosy to some extent.

However, unlike Rutie, Rosy doesn't expose the lower-level C bindings. The reasoning is that if certain functionality is missing from Rosy, it should be added to the core library by either requesting it through an issue or submitting a pull request with an implementation.

Also, unlike Rutie, Rosy marks all exception-throwing functions as unsafe. Not handling a Ruby exception from Rust-land results in a segmentation fault. One of the major reasons that some people choose to write Rust over C is to get away from these. The Rust philosophy is that safe code should not be able to trigger a segmentation fault. Just like with Rutie, Rosy allows Rust code to be protected against raised exceptions.


Rosy is a part of the Ocean package manager.

Support oceanpkg on Patreon

And it is developed by the following individual contributors:


This project is made available under either the conditions of the MIT License or Apache License 2.0 at your choosing.


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