Project Setup

A mixed Rust/Clojure project needs three things: a cljrs.edn that points to the Rust crate, a Cargo.toml with the right crate type, and a cljrs_init entry point.

Directory layout

my-project/
├── cljrs.edn          # Clojure project config (source paths, :rust key)
├── Cargo.toml         # Rust crate manifest
├── src/
│   ├── lib.rs         # Rust source — defines cljrs_init and native fns
│   └── main.cljrs     # Clojure entry point

The Rust crate and the cljrs.edn file can live in the same directory (:crate ".") or in a subdirectory (:crate "native").

cljrs.edn

Add a :rust map to the top-level config:

{:paths ["src"]

 :rust {:crate "."                       ; path to Cargo.toml directory
        :init  "my_project::cljrs_init"} ; Rust path to the init function
}

Cargo.toml

The user crate must be a library with cdylib output (for interpreter-mode dynamic loading) and, optionally, rlib output (for AOT static linking):

[package]
name    = "my_project"
version = "0.1.0"
edition = "2024"

[lib]
crate-type = ["cdylib", "rlib"]

[dependencies]
cljrs-interop = { path = "/path/to/cljrs/crates/cljrs-interop" }

Note: cdylib produces the .so/.dylib/.dll loaded by cljrs run. rlib allows cljrs compile to link the crate statically into the AOT binary. Both can coexist in crate-type.

The cljrs_init entry point

The init function receives a *mut Registry pointer and registers all native functions. It must have C linkage so the dynamic linker can find it by name:

#![allow(unused)]
fn main() {
use cljrs_interop::{Registry, wrap_fn1, wrap_fn2};

#[no_mangle]
pub extern "C" fn cljrs_init(registry: *mut Registry) {
    let r = unsafe { &mut *registry };

    r.define("my.project/greet",
        wrap_fn1("greet", |name: String| {
            Ok::<String, String>(format!("Hello, {name}!"))
        }));

    r.define("my.project/add",
        wrap_fn2("add", |a: i64, b: i64| Ok::<i64, String>(a + b)));
}
}

The function name in :rust :init ("my_project::cljrs_init") must match the Rust function name used in #[no_mangle] (cljrs_init). The crate prefix (my_project) is used when generating the AOT harness; it must match the [package] name in Cargo.toml with hyphens replaced by underscores.

Calling native functions from Clojure

Native functions registered under "my.project/greet" are visible in Clojure as my.project/greet. No require is needed unless you want a namespace alias:

; Direct qualified call
(my.project/greet "world")       ; => "Hello, world!"

; With a require alias
(ns my.app
  (:require [my.project :as native]))

(native/add 3 4)                 ; => 7

The namespace my.project is created automatically when cljrs_init is called; you do not need to create or load a Clojure file for it.