Preserve cause-chain of errors propagated from rust -> c++ -> rust

Description

use anyhow::{anyhow, Result};

#[cxx::bridge]
mod ffi {
    extern "C" {
        include!("repro.h");
        fn cthing() -> Result<()>;
    }

    extern "Rust" {
        fn rustthing() -> Result<()>;
    }
}

fn rustthing() -> Result<()> {
    Err(anyhow!("no such file or directory").context("failed to open: /nonexist"))
}

fn main() -> Result<()> {
    ffi::cthing()?;
    Ok(())
}

// repro.cc

void cthing() {
  rustthing();
}

Error: failed to open: /nonexist

Contributor guide

Tech stack

rustcpp

Domain

backenddeveloper experience

Issue type

bug

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

4

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

3-5 days

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 programmingC++ basicsUnderstanding of cxx FFIKnowledge of anyhow crate

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

25

Research direction

Investigate how cxx currently converts Rust anyhow errors into C++ exceptions and back. Look at the code in cxx/src/cxx.cc or similar files that handle the FFI error conversion. Examine any linked issues or comments for prior discussion. The goal is to preserve the full error chain across the C++ boundary.