flapigen

In the first step you should choose "main" crate that would be the bridge between Rust and other language. It should have crate-type cdylib or staticlib, see suitable Rust book's section for a detailed description.

For example:

[lib]
name = "cpp_example_rust_part"
crate-type = ["cdylib"]

Then you should create build.rs inside this crate. For example:

//build.rs
use flapigen::{CppConfig, CppOptional, CppStrView, CppVariant, LanguageConfig};
use std::{env, path::Path};

fn main() {
    let out_dir = env::var("OUT_DIR").expect("no OUT_DIR, but cargo should provide it");
    //ANCHOR: cpp_config
    let cpp_cfg = CppConfig::new(
        // ANCHOR: cpp_output
        Path::new("..").join("cpp-part").join("rust-api"),
        // ANCHOR_END: cpp_output
        "rust".into(),
    )
    .cpp_optional(CppOptional::Boost)
    .cpp_variant(CppVariant::Boost)
    .cpp_str_view(CppStrView::Boost);
    //ANCHOR_END: cpp_config
    let swig_gen = flapigen::Generator::new(LanguageConfig::CppConfig(cpp_cfg));
    swig_gen.expand(
        "c++-api-for-rust",
        // ANCHOR: rust_input
        Path::new("src/cpp_glue.rs.in"),
        // ANCHOR_END: rust_input

        // ANCHOR: rust_output
        &Path::new(&out_dir).join("cpp_glue.rs"),
        // ANCHOR_END: rust_output
    );
    println!(
        "cargo:rerun-if-changed={}",
        Path::new("src").join("cpp_glue.rs.in").display()
    );
}

Here you instruct flapigen to generate C++/Rust code from "src/cpp_glue.rs.in" as input:

        Path::new("src/cpp_glue.rs.in"),

and we specify our output, Rust file:

        &Path::new(&out_dir).join("cpp_glue.rs"),

directory for C++ files:

        Path::new("..").join("cpp-part").join("rust-api"),

You can find a detailed description of code generation in the suitable Cargo book section.

Then you should create a file for the "foreign" language API description. For example:

use super::{f2, Foo};

foreign_class!(class Foo {
    self_type Foo;
    constructor Foo::new(_: i32) -> Foo;
    fn Foo::set_field(&mut self, _: i32);
    fn Foo::f(&self, _: i32, _: i32) -> i32;
    fn f2(_: i32) -> i32;
});

And connect the generated code with your crate exactly how as in the Cargo book :

// src/lib.rs
mod cpp_glue;
pub use crate::cpp_glue::*;

// src/cpp_glue.rs
include!(concat!(env!("OUT_DIR"), "/cpp_glue.rs"));

Do not forget to add flapigen as a dependency into [build-dependencies] section of your crate's Cargo.toml file and you are ready to go.