swiftc + Cargo

One approach to building Rust and Swift is to use Cargo to compile your Rust code and swiftc to compile your Swift code.

In this case, either Cargo needs to link to a compiled Swift native library, or swiftc needs to link to a compiled Rust native library.

We'll use an example project to explore both of these approaches.

Project Setup

cargo new --lib swift-and-rust
cd swift-and-rust

# Cargo.toml

[build-dependencies]
swift-bridge-build = "0.1"

[dependencies]
swift-bridge = "0.1"

// src/main.rs
fn main() {
    swift_and_rust::print_hello_swift();
}

#![allow(unused)]
fn main() {
// src/lib.rs

pub use ffi::print_hello_swift;

#[swift_bridge::bridge]
mod ffi {
    extern "Rust" {
        fn print_hello_rust();
    }

    extern "Swift" {
        fn print_hello_swift();
    }
}

fn print_hello_rust() {
    println!("Hello from Rust")
}
}

// main.swift
run()

// lib.swift
func print_hello_swift() {
    print("Hello from Swift")
}

func run() {
    print_hello_rust()
}

# bridging-header.h

#ifndef BridgingHeader_h
#define BridgingHeader_h

#include "./generated/SwiftBridgeCore.h"
#include "./generated/swift-and-rust/swift-and-rust.h"

#endif /* BridgingHeader_h */

mkdir generated

Swift links to a Rust native library

Add the following to your Cargo.toml

# Cargo.toml

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

// build.rs

use std::path::PathBuf;

fn main() {
    let out_dir = PathBuf::from("./generated");

    let bridges = vec!["src/lib.rs"];
    for path in &bridges {
        println!("cargo:rerun-if-changed={}", path);
    }

    swift_bridge_build::parse_bridges(bridges)
        .write_all_concatenated(out_dir, env!("CARGO_PKG_NAME"));
}

# build-swiftc-links-rust.sh

#!/bin/bash
set -e

cargo build --target x86_64-apple-darwin
swiftc -L target/x86_64-apple-darwin/debug/ -lswift_and_rust -import-objc-header bridging-header.h \
  main.swift lib.swift ./generated/swift-and-rust/swift-and-rust.swift ./generated/SwiftBridgeCore.swift

chmod +x build-swiftc-links-rust.sh
./build-swiftc-links-rust.sh
./main
# The output should be "Hello from Rust"

Rust links to a Swift native library

Unlike when we had swiftc linking in the Rust code, you do not need to set the crate-type when you have Cargo linking in the Swift code.

// build.rs
fn main() {
    println!("cargo:rustc-link-lib=static=my_swift");
    println!("cargo:rustc-link-search=./");
}

# build-swift-static-lib.sh

#!/bin/bash
set -e

# Generate the bridging headers.
# Multiple files can be supplied with "-f" flag, e.g. "-f file1 -f file2".
# Substitute <package name> for the crate name in your Cargo.toml file.
swift-bridge-cli parse-bridges --crate-name <package name> -f src/lib.rs -o generated

swiftc -emit-library -static -module-name my_swift -import-objc-header bridging-header.h \
  lib.swift ./generated/swift-and-rust/swift-and-rust.swift

chmod +x build-swift-static-lib.sh
./build-swift-static-lib.sh

cargo build
./target/debug/swift_and_rust
# The output should be "Hello from Swift"