The `cxx` crate is not used for building the Rust compiler itself; it is a library for safe Rust/C++ interoperability in application code. To use `cxx` in your project, add the dependency to your `Cargo.toml` and define your C++ bindings in a `#[cxx::bridge]` module.

How to Use the cxx Crate for Rust/C++ Interop

The cxx crate is not used for building the Rust compiler itself; it is a library for safe Rust/C++ interoperability in application code. To use cxx in your project, add the dependency to your Cargo.toml and define your C++ bindings in a #[cxx::bridge] module.

[dependencies]
cxx = "1.0"

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn rust_function() -> i32;
    }
    unsafe extern "C++" {
        type CppType;
        fn cpp_function() -> i32;
    }
}

fn rust_function() -> i32 {
    42
}

fn main() {
    let result = rust_function();
    println!("Rust result: {}", result);
}

You must also provide the corresponding C++ implementation file (e.g., ffi.cpp) and ensure your build script (build.rs) invokes cxx_build::bridge!() to generate the glue code.

The cxx crate acts as a safe bridge between Rust and C++ code, allowing them to call each other's functions without manual memory management. It is used when you need to reuse existing C++ libraries or share logic between the two languages in a single application. Think of it as a translator that ensures both languages understand each other's data formats safely.