To call Rust from Ruby with Magnus, build the Rust code as a native extension, mark its Ruby-loadable initializer with #[magnus::init], and register Rust functions, methods, or wrapped types there. Package the compiled extension with a Ruby gem so Ruby can load it through the gem’s normal API.
How the Ruby-to-Rust path works
Magnus is a Rust library for writing Ruby extension gems in Rust. Ruby remains the host: it loads the native extension, Magnus initializes the Ruby-facing API, and calls into that API reach Rust code. This is different from embedding Ruby in a Rust executable, where Rust is the host and the deliverable is a Rust program rather than a Ruby gem.
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The basic flow is:
- Configure a Rust library to compile as a dynamic library.
- Implement Rust logic and mark an initializer with
#[magnus::init]. - Register the Rust functions or methods as Ruby names inside the initializer.
- Package the native library as a gem extension and load it from Ruby.
Build a minimal Magnus extension
1. Configure the Rust library
In Cargo.toml, configure the library crate type as cdylib and add Magnus as a dependency. The Magnus repository’s getting-started example uses magnus = "0.8", while the API documentation surfaced here identifies version 0.9.1. Choose a version intentionally and follow documentation matching the version in your project rather than assuming snippets are interchangeable.
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[lib]
crate-type = ["cdylib"]
[dependencies]
magnus = "0.8"The version line above reflects the repository example, not a universal recommendation. Check the Magnus repository and the API documentation for the version you select.
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2. Implement and register a Ruby function
Write the Rust function, then register it in the extension initializer. Magnus’s getting-started guide uses a distance function as an example: the Rust function accepts two coordinate tuples and returns a floating-point value, and function!(distance, 2) binds it as a Ruby function.
#[magnus::init]
fn init() {
// Register Rust functions and Ruby-facing types here.
}The snippet illustrates the initializer pattern; it is not a complete, version-independent extension. Consult the guide for the exact registration API and surrounding setup required by your Magnus version.
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Ruby invokes the function marked with #[magnus::init] when it loads the extension. Use that initializer to define the module, class, or functions that Ruby code will call.
Choose the Ruby API shape
Expose a plain function
Use Magnus’s function! binding when the operation fits a module-level Ruby function. The binding connects the Rust function to a Ruby-visible name and specifies its argument count.
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Expose an instance method
Use method! when the operation belongs on a Ruby object. Account for Ruby’s self in the Rust-side function signature: the method binding includes the receiver in addition to the method’s explicit arguments.
Return Rust-backed objects to Ruby
For object-oriented APIs, Magnus can expose Rust structs or enums as Ruby objects. Its #[magnus::wrap] attribute provides a convenience route; implementing the TypedData trait offers more control over the wrapped type. Choose based on how much customization the object’s Ruby representation and lifecycle need.
Handle conversions and Ruby errors
Magnus supports conversions between common Ruby and Rust types. A Rust function can return a Result so errors cross the boundary as Ruby exceptions; incompatible arguments can also result in Ruby-style type or argument errors. Decide which failures should be raised to Ruby and propagate or handle them deliberately.
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Protect Ruby objects from garbage-collection hazards
Ruby’s garbage collector must be able to find Ruby objects that Rust code is using. Magnus documents a critical rule: Ruby objects in Rust must remain on the stack so the collector can reach them. Storing Ruby objects in heap-allocated structures such as Vec, HashMap, or Box can hide them from the collector and create memory-safety problems.
Rust’s type system and borrow checker do not enforce this rule for you. Follow Magnus’s documented lifetime and rooting requirements, especially when a Rust object needs to retain a Ruby value beyond the immediate call.
Package the extension as a gem
A working Rust library still needs Ruby-native-extension packaging so it can be compiled and loaded as part of a gem. RubyGems documents native extensions, and Magnus’s repository recommends using rb_sys with rake-compiler for gem packaging. The gem’s Ruby code then requires the resulting native library.
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Native compilation and loading depend on the versions and target platforms involved. Follow the current instructions for your chosen Magnus, Ruby, and packaging-tool versions; the published guidance cited here does not establish one build recipe that works across every Ruby implementation, operating system, or target.
Keep the two interop directions distinct
| Goal | Host runtime | Typical deliverable |
|---|---|---|
| Call Rust code from Ruby through Magnus | Ruby loads a native extension | A Ruby gem containing or building the extension |
| Call Ruby code from Rust | Rust embeds Ruby | A Rust executable using an embedding workflow |
If your application is a Ruby gem that needs a Rust implementation behind its Ruby API, use the extension route described above. Embedding Ruby is the inverse use case, not another way to load a Magnus extension into Ruby.
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