To call Rust from Ruby with Magnus, build the Rust code as a native extension, mark its Ruby load entry point with #[magnus::init], and register Rust functions or methods there. Package the compiled extension with a Ruby gem so Ruby can load it through the gem’s ordinary API.
How the Ruby-to-Rust call works
Magnus is a Rust library for writing Ruby extension gems in Rust. The Ruby process remains the host: it loads a compiled native library, calls its Ruby-facing functions or methods, and Magnus converts values and errors across the boundary. This is different from embedding Ruby in a Rust executable, where Rust hosts and initializes the Ruby runtime. See the Magnus project guide and API documentation.
Set up a Rust library for Ruby
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In
Cargo.toml, configure the library as a dynamic library by setting[lib]withcrate-type = ["cdylib"]. -
Add Magnus as a dependency. The repository getting-started guide shows
magnus = "0.8", while the API documentation surfaced here identifies version 0.9.1. Select a version deliberately and follow its matching documentation; do not assume an older example is unchanged.Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy.#1 Best Overall
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Write a Rust initialization function and mark it with
#[magnus::init]. Ruby calls this entry point when it loads the extension, and the initializer is where you define the Ruby-facing API. -
Package the native library as part of a gem. RubyGems describes native extensions as code compiled during installation; Magnus recommends using
rb_syswithrake-compilerfor gem packaging. Follow the current instructions for the versions and target platform in your project. See the RubyGems native extensions guide.Rank #2
Expose Rust functions and methods
Register a Ruby function
Use Magnus’s function! macro to expose a Rust function under a Ruby name. The project guide’s getting-started pattern registers a Rust distance function with function!(distance, 2), declaring two Ruby arguments. The function can contain ordinary Rust logic; the initializer connects it to Ruby.
Register an instance method
Use method! when the Ruby API should expose a method. The Rust-side signature accounts for Ruby’s receiver, self, as an additional argument. Match the binding’s arity to the callable’s signature and the Ruby arguments it accepts.
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Wrap Rust-backed objects
If Ruby needs to hold Rust values between calls, Magnus can expose Rust structs or enums as Ruby objects. The #[magnus::wrap] attribute is a convenience route; implementing the TypedData trait offers more customization. In either case, the wrapper lets values pass between Ruby and Rust. Choose based on the Ruby API and object-lifecycle needs rather than wrapping every Rust type by default.
Handle conversions and errors at the boundary
Magnus supports conversions for common Ruby and Rust types. A Rust function can return Result so failures cross the boundary as Ruby exceptions; incompatible argument types can also produce Ruby-style type or argument errors. For Ruby methods without a direct C API counterpart, Rust code can call Ruby through Magnus’s funcall. Handle or propagate the resulting magnus::Error rather than silently discarding it.
Keep Ruby objects visible to garbage collection
When Rust code holds Ruby objects, those objects must remain reachable to Ruby’s garbage collector under Magnus’s documented lifetime and rooting rules. In particular, placing Ruby objects in heap-allocated structures such as Vec, HashMap, or Box can make them invisible to the collector and create memory-safety risks. Rust’s type system and borrow checker do not enforce this rule. Consult Magnus’s safety documentation and use its documented APIs and rules whenever Ruby objects need to outlive a call or be retained.
Check version and platform compatibility
The repository’s getting-started example and the surfaced API documentation refer to different Magnus versions (0.8 and 0.9.1, respectively). Keep the dependency, macros, and code aligned with the version in your Cargo.toml. The cited sources do not establish one build recipe that is guaranteed across Ruby implementations, operating systems, or build environments, so verify the current packaging and compilation instructions for your target.
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