Support passing function pointers across the FFI

Description

#[cxx::bridge]
mod ffi {
    extern "Rust" {
        fn f(arg: u8, callback: fn(u8));
    }
}

fn f(arg: u8, callback: fn(u8)) {
    callback(256 - arg);
}

Contributor guide

Tech stack

rustcpp

Domain

backendapi

Issue type

feature

DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.

5

Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.

over 1 week

Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.

stale

ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.

clear

Prerequisites

Rust FFI knowledgeC++ interopcxx crate internals

Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.

10

Research direction

The issue requests adding support for function pointer types in cxx bridge. The current implementation likely does not handle `fn(...)` parameters. To start, examine how cxx currently handles other types like references and boxes in the code generation. Look for existing handling of function pointer types in the source code (e.g., in `src/`). Consider how closures or extern functions are currently supported. The PRs and comments may provide guidance. Focus on the parser and code generation for the `extern "Rust"` block. Also check for any previous attempts or discussions in the comments.