scylladb/docs/dev/rust.md

Rust and C++

Rust introduces many useful features that are missing in C++. This document shows how to use them in Scylla.

Using Rust in Scylla

To create a Rust package new_pkg and use it in a C++ source file:

1. Create a new package in the rust directory using cargo new new_pkg --lib
2. Modify crate type to a static library by adding

[lib]
crate-type = ["staticlib"]

to new_pkg/Cargo.toml 3. Add "new_pkg", to the workspace members list in Cargo.toml 4. Write your Rust code in new_pkg/src/lib.rs 5. To export a function fn foo(x: i32) -> i32, add its declaration as follows to the same file

#[cxx::bridge(namespace = "rust")]
mod ffi {
    extern "Rust" {
        fn inc(x: i32) -> i32;
    }
}

6. Add "rust/new_pkg/src/lib.rs" to the rusts list in configure.py
7. Include the rust/new_pkg.hh header and use rust::foo() in the selected c++ file.

Rust interoperability implementation

Using Rust alongside C++ in scylla is made possible by the Rust crate CXX. The cxx::bridge macro, together with mod ffi and extern "Rust" mark items to be exported to C++. During compilation, a compatible version is generated with temporary names. It can be later linked with a header file, generated from the source code using cxxbridge command. The header exposed all items with original names and can be included like any other c++ header.

Compilation is managed by cargo. Like in any Rust project, modules added to Scylla can be fully customized using corresponding Cargo.toml files. Currently, all modules are compiled to static libraries (TODO: dynamic or flexible)